Coherence is a way of talking about how ideas in a sentence or in a group of sentences fit together. In linguistics, it's important that ideas are connected closely together, so that someone who is listening or reading can understand what the speaker or writer is trying to say. For example, if someone said, "I like cheese, oranges, and to watch TV," that wouldn't make a lot of sense because those things aren't really related. So instead they might say, "I like to snack on cheese while I watch TV." This makes more sense and is more coherent.