about the definition, but basically, in a two-player zero-sum game, if you're playing the Nash equilibrium, you are guaranteed to not lose an expectation. Now, it's not always easy to find a Nash equilibrium, but it's always guaranteed to exist and a finite two-player zero-sum game. So, for example, in rock, paper, scissors, the Nash equilibrium is to this mix randomly between rock, paper, and scissors, with equal probability, because if you do that, then no matter what your opponent does, you will not lose an expectation.