A dependent clause has a subject and a verb; a phrase does not.
…after lunch (phrase)
…after we ate lunch (clause)
…since Friday (phrase)
…since I left Friday (clause)
…before the session (phrase)
…before the session ended (clause)
The difference matters. For example, at the beginning of a sentence, the phrases above do NOT take a comma; the clauses do.
…After lunch we got down to the business at hand.
…After we ate lunch, we got down to the business at hand.