Bet/call is when you bet with the intention of calling your opponent's raise. The bet/call can be made for several reasons. First, against a very aggressive player who bluffs very often, betting/3beting rather than bet/calling may cause him to abandon his bluff and fold. You don't want that. Also, by simply calling your bet, you are hiding the strength of your hand. We can also bet/call because our hand is not strong enough to bet/3bet, but too strong to bet/fold. Let's imagine a color draw with overcards for example.
Specific bet/call situations
As mentioned in the paragraph above, there are many bet/call situations. Let's say you have a pair of 9s in your hand on an all-in 7-8-9 flop. You bet and your opponent (a very conservative player who is known to be passive) raises your bet on the flop. Here, you are often against straights or flushes. The bet/call option would accomplish nothing, since by raising his raise you would be folding the weakest part of your opponent's range and getting some action against his dominating hands. The bet/call option is therefore the obvious choice.