An Utterance is the spoken equivalent to a sentence.
As a sentence is bounded by a capital letter and a period or full stop (or other punctuation) so an utterance is often (though not always) bounded by silence or a breath or pause.
Having said this, there is no precise definition of utterance that linguists use.
Written sentences tend to be well formed in terms of grammar. We usually write them bearing in mind the generally accepted rules of grammar, e.g. we include a verb, we group words in the right way, we have subject-verb agreement and so no.
However utterances often break these rules:
Written Sentence The cat sat on the mat.
Spoken Utterance hey! did you, er there was this, um, cat it sat on um, what’s the word? oh yea mat