Game theory optimal (GTO) play is a poker strategy that involves making decisions based on mathematical models and probability theory, with the goal of maximizing expected value and minimizing exploitability. GTO play involves balancing ranges, mixing up actions to make them unpredictable, and avoiding biases and exploitable patterns. It also requires considering the opponent's possible ranges and reactions, and adjusting one's strategy accordingly. By playing GTO, a player aims to make their decisions unexploitable, making it difficult for opponents to gain an edge.