Linguists refer to utterances when they don't want to get bogged down into arguments about whether they're talking about a sentence, a phrase, a word, or something else. When you're learning an unfamiliar language, you might not know the structure of something you just heard something say, so rather than inaccurately calling it a "word" or a "sentence", you just refer to it as an utterance. Later, you can figure out how many distinct words are in that utterance. For now, you're busy taking note of what you heard, in what context, and what meaning it might represent.