A phrase is a word or group of words functioning as a unit within the structure of a sentence. Phrases have a hard time conveying information all on their own because they lack either a subject or a verb. They are snippets of ideas that could be actions if only they could find a clause to stand by, to modify.
Phrases behave like adjectives (modifying nouns), adverbs (modifying verbs, adverbs, and adjectives), or even a noun. They embellish sentences and create a variety of sentence structures, which allows the sentence to convey more specific or interesting information. They allow the sentence to do more.
If a sentence is like the on-field players of a soccer team, phrases would be the defensive players working as a unit within the larger structure of the field.
To tell whether you’re looking at a phrase or a clause, check if there is both a subject and a verb. Only a clause will contain both, that's why you'll catch more going on in a clause.
Phrases can be prepositional: The flick of the flash over the threshold is what scared him the most.
Phrases can be adverbial: After the dog left rather quickly, I waited most anxiously for him to return.
Phrases can be adjectival: The audience was very happy that the 5th grade production of Helen Keller was over, but the kids were more tired than anything.