Israeli Prime Minister Menahem Begin outlined to President Carter yesterday offers that were described as significant concessions to restore Arab authority on the West Bank of the Jordan River, in the Gaza Strip, and in the Sinai Desert.

To maintain the momentum of the drive for peace, Begin and Egyptian President Anwar Sadat will meet a second time, within two weeks. it was disclosed. Begin's office in Tel Aviv confirmed the new meeting, but not the location. Sadat indicated it would be in Egypt, and diplomatic sources suggested the site may be Ismalia, on the Suez Canal.

President Carter also may meet with Sadat in the coming weeks, the Egyptian leader told reporters in Cairo, saying such a meeting "is not excluded." White House officials said so far, however, that there are "no plans" for an early Carter-Sadat meeting.

In a rush of interlocking developments, Carter met with Begin for two hours yesterday at the White House, conferred with Sadat by telephone, and scheduled a second meeting with Begin starting at 7 o'clock tonight.

Carter told Begin, the White House said afterward, that "the United States is convinced that the course of direct negotiations on which Prime Minister Begin and President Sadat have embarked offers a unique opportunity for peace."

Begin "outlined proposals concerning the future relations between Egypt and Israel and a process for resolving the issue of Palestinian Arabs," the White House said in its statement.

The terminology of "Palestinian Arabs" is an elliptical reference employed by Begin on the key Arab demand for a Palestinian state in the West Bank and the Gaza Strip, territory occupied by Israel in the 1967 Arab-Israeli war. Begin's government rejects the demand for a Palestinian state, but it is now offering Arab autonomy in these regions, in a complex formula that would preserve Israeli security interests in these areas.

No details of the Begin offer were officially disclosed by either side yesterday. Carter, it was annouced, "thanked the prime minister for his thoughts" and "promised to give them serious consideration."

At this time, White House press secretary Jody Powell said, the United States is avoiding "characterizing" the offer.

However, senators who lunched with Begin at Blair House said he had presented promising and "constructive" proposals that can serve as a basis for peace settlements with Egypt, Jordan, and presumably Lebanon as well. There are no current Israeli proposals involving Syria, diplomatic sources said, because of Syria's hostile posture toward the Sadat-Begin initiatives.

What Begin is offering, Sen. Jacob K. Javits (R-N.Y.) said, represents "a real breakthrough in efforts to bring about a peace . . . a credible basis for negotiation - designed sincerely to take Sadat at his word" to achieve peace.

Sen. Richard B. Stone (D-Fla.), chairman of the Senate Foreign Relations Subcommittee on the Middle East and a champion of Israel, said this process "will take some time and will involve a number of steps."

The Begin offer, Stone said, "can provide Sadat with a negotiating basis for Egyptian and other Arab issues, particularly Palestinian issues - the West Bank and Gaza." Begin, Stone said, was "quite happy" with Carter's attitude toward his proposals.

Sen. Henry M. Jackson (D-Wash.) said they are "most impressive proposals," but "this is not the time to pop off" about the details.

When they are unveiled, Jackson said, "we can all say we are really moving down the road to peace in the Middle East for the first time since 1947," He said, "I think moderate Arabs, Egyptians, sensible Palestinians will buy it," but "the radical Arabs and Palestinians won't buy anything, anyway."

"As Christians approaching the Christmas season," Jackson said, "we can all be thankful to a Moslem and a Jew," referring to Sadat and Begin.

Carter and Begin spent an hour in private talks, before being joined for another hour by other officials including Secretary of State Cyrus R. Vance, Vice President Mondale, presidential national security affairs adviser Zbigniew Brzezinski, and Begin's aides. The Israelis included Aahron Barak, Begin's chief legal adviser, and Israeli Ambassador Simcha Dinitz.

Vance and Barak, plus other aides, later met at the State Department for 80 minutes of discussion on what diplomatic sources said were numerous complex legal issues. "This is not a simple proposal," said one source.

An Israeli general was seen carrying a roll of maps out of the White House meeting and Powell said "charts "charts and papers" were used in Begin's presentation. Powell described the discussions as "serious and friendly." An Israeli spokesman said there was "a very good atmosphere."

Powell said Carter telephoned Sadat in Cairo after the meeting to discuss what Begin presented. Sadat's reaction was not disclosed. It is clear there is bound to be extensive bargaining over the Israeli proposals.

The White House statement said Carter and Begin are discussing "underlying principles which could guide future negotiations," leading to "the broader goal of negotiating a comprehensive peace" between Israel and all the Arab nations.

The core of the Begin offer on the Palestinian-West Bank issue is reported to be a plan for providing Arab autonomy for the West Bank and Gaza Strip, while retaining Israeli security rights. This is a variation on previous proposals by the former Labor Party government. Begin's Likud Party, which had been totally opposed to surrendering any authority in the West Bank, Judea and Samaria of biblical days, is now reportedly showing more specific flexibility than the previous government.

According to the Israeli newspaper Maariv, Begin's proposal would establish an autonomous region including Judea, Samaria and Gaza. Israeli sovereignty would not extend to this region, nor would any other sovereignty, but it would be linked to Jordan, in a manner unspecified. Israel would have the right to maintain military forces in the region, through arrangements endorsed by international agreements, it was reported.

The plan also would return almost all of the Sinai Desert to Egypt. Most of that region would be demilitarized, with a U.S. presence and Israeli-Egyptian personnel at early warning stations. Officials decline to confirm or deny this report and others resembling it.

Sadat, in an ebullient mood, told reporters in Cairo yesterday as he left a mosque after prayer that he planned to meet Begin before the end of this year - within two weeks. Askes where, Sadat replied, "Why not Egypt?"

The Egyptian leader then said it could not be excluded that he would meet both Carter and Begin before the year is out. Begin's office in Tel Aviv later confirmed the second Sadat-Begin meeting, first proposed during Sadat's sensational trip to Jerusalem last month, saying the next meeting would take place "somewhere in the Middle East."

A Carter-Sadat meeting conceivably could take place at the end of Carter's planned trip abroad, which begins Dec. 29, but which now has only one Middle East stop, in Saudi Arabia.