The part of above sentence “near a wall” is a phrase because it does not contain subject and verb, and does not express a complete idea.
A phrase does not include both subject and verb at a same time and does not make a complete sense, hence a phrase cannot stand as a sentence on its own.
If a group of words include both subject and verb then it becomes a clause, so the difference in a clause and a phrase is that a clause contains subject and verb but a phrase does not contain subject and verb.
Here are some examples of phrases.