GTO stands for Game Theory Optimal, the optimal way to play a hand or street according to Game Theory. Game Theory, according to Wikipedia, is the study of mathematical models of strategic interaction between rational decision-makers.
Many of the top poker professionals study Game Theory to understand the rationale behind poker. While a GTO approach to the game is unexplitable, it's not always the optimal way of playing since many opponents don't play optimal themselves, thus making it optimal to divert from a GTO approach to extract maximum value.
Podcast host Joey Ingram uses the term GTO for far more than just poker hands and poker situations, he uses the term to put value on life decisions and how close they are to being optimal given a rational train of thoughts.