For example, the sentence "I ate a red apple" entails the sentence "I ate an apple", because if it's true that I ate a red apple then it must also be true that I ate an apple. More generally, some proposition A entails some proposition B if A being true guarantees that B must also be true. It should be clear that an entailment is a truth condition: for the sentence "I ate a red apple" to be true, one of the things that must be true (i.e., one of the truth conditions) must be that I ate an apple.