Definition of bet/fold
The term bet/fold is used in a number of scenarios. Basically, what we mean by bet/fold is that we bet with the intention of folding if our opponent raises us. You can bet a flop with the idea that if your opponent raises you, your next play will be to fold (so here you bet/fold). We can bet/fold because our bet was originally a bluff. We can also bet/fold because the player who raises us is much too passive. Against a passive player, without the nuts and facing a raise, it is often a good option to bet/fold.
The bet/fold on the river
A particularly good place to make a bet/fold is on the river in position. Usually, when our opponent checks on the river after playing the hand passively, we can expect that he doesn't have a very strong hand. If we have top pair, we can make a classic bet/fold, keeping in mind that the player will often call the bet with worse, but if he were to raise, our hand would no longer be good and it would become a bet/fold. The term refers to the idea of thinking several moves ahead.