A strategy built so that no opponent, however well they read you, can profit by adjusting to it. The name comes from game theory: in a two-player zero-sum game there exists an equilibrium where neither side can improve by unilaterally changing course, and a player following it is unexploitable.
What that buys you is a floor, not a ceiling. An equilibrium strategy does not try to beat anyone in particular — it declines to be beaten. Against a strong, unfamiliar, or observant opponent that is exactly what you want. Against someone who folds far too often or calls with anything, it deliberately leaves money on the table, because punishing those mistakes means creating exploitable patterns of your own.
In practice the balance shows up as mixed frequencies: the same holding bet sometimes and checked other times, in proportions that keep your betting range from being read. That is why solver output is a table of frequencies rather than a list of correct plays.
Two cautions are worth carrying. First, the unexploitability guarantee is a two-player result; in a multiway pot the theory is far less clean, and confident claims about the GTO play three-handed deserve suspicion. Second, an equilibrium strategy is a benchmark, not a plan — most of the money in a soft game comes from deviating toward exploitative play once a tendency is clear, and returning to the baseline when it is not.
Study it to know where the centre is. Leave it to make money.