GTO - Game Theoretical Optimum

This concept from game theory refers to a play that can be considered the optimal strategy in terms of a Nash equilibrium.

In reference to poker this means: If you are playing an optimal strategy, it does no longer matter what your opponent does since his play against you will never have a positive expected value.

However, this does not mean that the EV of an optimal strategy always equals the maximum EV possible. For this, you would have to exploit the specific leaks in your opponent's game, even if such a play might differ from the optimal strategy.

One example for this are Nash ranges, as they provide you with optimal pushing ranges or calling ranges in a given situation.