This lesson uses the stag hunt to introduce the concept of pure strategy Nash equilibrium (PSNE).
Holding all other players' actions constant, a "best response" is the most profitable move a particular player can make.
A game is in Nash equilibrium when all players are playing best responses to what the other players are doing.
Put differently, a Nash equilibrium is a set of strategies, one for each player, such that no player has incentive to change his or her strategy given what the other players are doing. (This is the definition of Nash equilibrium from lesson 1.3 of my textbook.)
Nash equilibria can be inefficient.
At least one Nash equilibrium exists for all finite games.
A game is finite if the number of players in the game is finite and the number of pure strategies each player has is finite. The stag hunt has two players, each of whom has two strategies. Therefore, it is a finite game.