A Nash equilibrium is a pair of strategies (or, one strategy for each player) such that no player can get a better result on average from deviating from it.
If one player isn’t playing a strategy that is part of any Nash equilibrium, then the best response might also not be part of any Nash equilibrium.
If all other players are playing a strategy from a given Nash equilibrium, then you can’t do better (in expectation) than you would if you were to play the strategy for you in that Nash equilibrium.
(A game may have multiple Nash equilibria. Possibly one such equilibrium could be better for you (or for everyone) than another.)
If one player isn’t playing a strategy that is part of any Nash equilibrium, then the best response might also not be part of any Nash equilibrium.
If all other players are playing a strategy from a given Nash equilibrium, then you can’t do better (in expectation) than you would if you were to play the strategy for you in that Nash equilibrium.
(A game may have multiple Nash equilibria. Possibly one such equilibrium could be better for you (or for everyone) than another.)