Definition of check/fold
A check/fold is when you check with the intention of folding against any bet. You can check/fold for many reasons. You can check/fold because your hand is poor or because the pot is multiway and you have a weak hand. Imagine that you have 3bet a hand like JJ, but you have received 4 calls. The flop comes A-K-2 all in hearts and you don't have the heart jack. This is a time when you think ahead and tell yourself that this is the ideal check/fold situation.
Another reason to check/fold
You shouldn't be afraid to check/fold. Some players are too proud to do so. If you have made a 3bet with 9-T in spades from the small blind against an over-opening button and the flop is not favorable and you are against a calling station player who never folds, rather than losing your entire stack trying to bluff him with limited success, perhaps you should just tell yourself that it is simply time to check/fold. In poker, you make money making good calls, but you also save money making good folds.