Game Theory Optimal (GTO) is the mathematically perfect way to play poker that makes it impossible to be exploited by opponents. In other words, if you play a perfect GTO strategy, your opponents could do no better than break even against you — and that would only be if they played a perfect GTO strategy themselves.
The actual GTO poker strategy is not yet known (and it may never be fully known due to the complexity of the game). However, certain aspects of the GTO strategy are known, such as using mixed strategies, balanced ranges, and multiple bet sizes in the same situation with different hands. Players who attempt to play a GTO style use these concepts as a part of their strategy.