SECRETARY OF STATE George Shultz heads off today for his first involvement in the high-profile, high-risk, high-voltage personal diplomacy that has become the principal means by which American policy has operated in the Middle East in recent years. His first priority is to close the small but agonizing lingering gap between Israel and Lebanon on the terms for Israel's withdrawal and his second is to salvage President Reagan's much-battered plan for a Palestinian-Israeli settlement. The importance of his mission, both to check the deterioration of the American position in the Middle East and to provide a badly needed tonic for the administration's foreign policy overall, can hardly be overstated.

As Mr. Shultz points out in an interview on our Topic A page today, Lebanon and Israel share the objective of making southern Lebanon secure and barring the reentry of PLO terrorists. The problem is working out the tactical arrangements. American diplomats in the area seem to have brought the talks to the point where it is not only sensible but necessary to put the secretary's prestige and negotiating experience into the breach.

Lebanon's readiness to wrap up negotiations is indisputable. Can the Israelis now see they do more for their border security and political interest by making an agreement that strengthens the Beirut government than by undermining it by demanding intrusive roles for their own troops and Lebanese clients? Israel's forces in Lebanon have become less a defense against terrorists than a magnet for them: the army is steadily taking more casualties than Israel suffered before it invaded Lebanon.

Secretary Shultz, in the interview, makes plain he has not written off Mr. Reagan's broader peace hopes. He observes, fairly, that King Hussein's refusal to join talks with Israel has shocked various Arab capitals to consider they might be "missing the boat." In a gesture to Jordan, Mr. Shultz now says that if King Hussein agrees to join the talks, the United States would not press him to "actually sit at the bargaining table until we can find some form of freeze" on Israeli settlements in the West Bank. In a gesture to Israel of potentially great significance, Mr. Shultz fills in a blank of the Reagan Sept. 1 peace plan and says it permits Jews to continue to live in the West Bank--"under the jurisdiction of whatever is the jurisdiction of that territory."

The meaning of these statements and of the decision to unsheath the secretary is that the Reagan administration is serious about retrieving lost ground. The pattern of Middle East diplomacy has always been volatile and unpredictable--in a word, maddening. Precisely this condition of fluidity creates an opening for American diplomacy. But the states of the area must understand that the Shultz trip is not so much a reprieve for the United States as for them.