President Reagan's description yesterday of what he would regard as a "fair agreement" with Moscow limiting medium-range missiles in Europe sounds similar to a package of ideas worked out in unauthorized secret discussions last summer by U.S. and Soviet negotiators.

Those informal discussions between chief U.S. negotiator Paul H. Nitze and his Soviet counterpart, Yuli A. Kvitsinsky, produced an "exploratory package" of ideas that were sent back to Washington and Moscow for study. Both the Soviets and the Reagan administration ultimately rejected them, although the administration for a time considered exploring them.

White House officials yesterday sought to play down any notion that Reagan's speech was meant to resurrect the ideas worked up by the two negotiators in Geneva.

But from what now has been revealed about the exploratory package, it bears a considerable resemblance to what Reagan laid out yesterday.

Although Reagan reiterated his "deep personal commitment" to his zero-zero plan for banning all medium-range missiles in Europe, he also laid out four principles that could form the basis of a possible compromise.

"The only basis on which a fair agreement can be reached is that of equality . . . " between the two superpowers, he said. Second, British and French missile forces, which are not part of the U.S.-Soviet talks in Geneva, cannot be considered in those talks.

In addition, the president said, the Soviets cannot be allowed merely to shift their missiles, which are mobile, from Europe to Asia. Finally, he said, any agreement must be verifiable.

The Nitze-Kvitsinsky package reportedly dealt with the equality issue directly, by limiting each side in Europe to 75 missile launchers. This allowed Moscow to keep 75 of its roughly 240 SS20 missiles now based in the European portions of the Soviet Union and aimed at western Europe. Each SS20 carries three warheads.

But it also left room for deployment of new U.S. cruise missile launchers, each of which comes with four missiles.

The British and French forces, which are independent and therefore not under NATO control, were not counted in the Nitze-Kvitsinsky package. Similarly, the two negotiators worked out a plan that would "freeze" the number of SS20s based in the Asian portions of the Soviet Union at 90, roughly the current level. This would seem to prevent any shift of missiles from Europe to Asia.

Very little is known about the verification provisions in the informal proposal.

The major American concession suggested by Nitze involved dropping the planned deployment of the Pershing II missile, while limiting cruise deployment to 75. The Pentagon strongly objected to dropping of the Pershing. That was one of the main reasons why the plan eventually floundered in Washington.