A fragment is a word/group of words that cannot fully complete a sentence. For example: but dogs are cool.

A phrase is a group of words without a subject, verb, or both. For example: Bodies cremated in a fire.

A dependent clause by itself is a fragment, but it is not a phrase, because it has a subject and verb.

You can fix a phrase by adding a subject/verb/both, and you can fix a fragment by adding it to an independent clause or adding a sentence component (like a subject).