The equilibrium strategy: one that loses no matter how the opponent plays.
It is a baseline that "doesn't lose," not the strategy that "wins the most." If the opponent is unbalanced, shifting to match that imbalance wins more. Example. If the opponent folds only 25% of the time, just dropping the baseline bluffs raises your EV.
At the table
Players say things like "the GTO play here is to bet 70% of the time and check 30%," meaning the equilibrium frequencies a solver computes.