Definition of bet/fold
Expression bet/fold is used in several scenarios. Basically, what is meant by bet/fold is that we bet (bet) with the intention of folding if our opponent raises us. We can bet a flop in bluff with the intention that if our opponent raises us, our next game will be flop (so here we have bet/fold). You can bet bet/fold because your bet was originally a bluff, or you can bet bet/fold because the player who raises you is too passive. Against a passive player, without the nuts and facing a raise, it's often a good option of bet/fold.
The bet/fold on the river
A particularly interesting place to do a bet/fold is on the river in position. Usually, when our opponent check on the river after playing the hand passively, we can expect that he will not have a very strong hand. If we have a top pair, we can make a classic bet/fold, keeping in mind that the player will often call the bet with worse, but that if he were to raise, our hand would not be good anymore and it would become a bet/fold. The expression refers to the idea of thinking several moves ahead.