Phrase is the shortest unit in english grammar. They are constructed with many words class words or one more words.
If you are asking it from the linguistic or grammatic perspective, then you possibly will profit from the definition I’m about to provide you with. (In case I understood your question wrong, then there is no need to read further. Specify your request or try to google it).
So, what is a Phrase?
A phrase is a group of words that stand together as a single unit, typically as part of a clause or a sentence.
Or, if speaking of the everyday speech, a phrase may be any group of words, often carrying a special idiomatic meaning; in this sense, it is roughly synonymous with expression.
In linguistic analysis, a phrase is a group of words (or possibly a single word) that functions as a constituent in the syntax of a sentence, a single unit within a logical hierarchy. A phrase typically appears within a clause, but it is possible also for a phrase to be a clause or to contain a clause within it.
There is a difference between the common use of the term phrase and its professional use in linguistics. In common usage, a phrase is usually a group of words with some special idiomatic meaning or other significance, such as "all rights reserved," "economical with the truth," "kick the bucket," etc. It may also be an euphemism, a saying or proverb, a fixed expression, a figure of speech, etc.
In the grammatical analysis, particularly in theories of syntax, a phrase is any group of words, or sometimes a single word, which plays a particular role within the grammatical structure of a sentence. It does not have to have any special meaning or significance or even exist anywhere outside of the sentence being analyzed, but it must function there as a complete grammatical unit.
It is common for a phrase not to contain a subject and verb and, consequently, cannot convey a complete thought. An expression contrasts with a clause. A clause does contain a subject and verb, and it can convey an entire idea.
This is only a short overview. If you are interested in a further reserch, I would suggest you to read these papers.