"Release the position !" is a stipulation used for many retro-problems. It also appears in other forms: "Solve the position !", "Open the North-East cage !"etc.
In essence, this stipulation asks you to find how a given (usually quite tangled) position can be reached legally. So that it really asks for a proof game but without insisting on giving all painful details of a proof game. You "only" have to find the tricky maneuver that is required to lock everything the way it appears in the diagram. The remaining details need not be spelled out.
With the word "release", the "Release the position !" stipulation emphasizes the kind of backward reasoning retro-solvers often use when they start from the diagram position and play backward moves (we call this "retracting") trying to reach the starting position. They usually stop this retraction process as soon as they are confident that no difficulty remains.
There are many such examples of this process among the detailed solutions of problems from this WWW RetroCorner, but here is one more:
Die Schwalbe, 1969
13+12. Release the position !
Cleraly, the point of almost all retrograde analysis is indeed to find how a given position can be reached legally, so that "Release ..." could be some kind of universal stipulation.
However, when they find it possible, retro-composers prefer more specific stipulations like "Mate in 1" that looks like a joke/trap, or e.g. "First move of the Bl. Queen?" that you cannot answer if you miss some part of the unlocking mechanism. In general, the "Release .. "stipulation is only used when nothing else will do.