A 'conceit' in poetry is a very elaborate metaphor. John Donne is peculiarly (particularly) famous for them. For example, in one poem he equates a man and a woman being bitten by the same flea to their having a romantic bond. If you are studying literature, it's a good idea to have a dictionary of literary terms. There are many words which have one meaning in a specific field and an entirely different meaning in the general language.

Hi There are two main meanings of the word 'conceit'. 1. Too much pride. This is by far the most common way to use this word in modern English. In this context a synonym is arrogance or hubris. 2. A thing that is created (or conceived). I've used the word 'thing' to give a deliberately vague definition because when this word first came into the language (around the time that John Donne was writing) it was used to describe all sorts of things (clothes, furniture, etc) that were made in a way that was clever or elaborate. So the synonym depends on the context if you are reading something that was written in the past. Gradually this use of the word has become rarer and rarer, and now it's most likely that you will only find it in modern English when someone is talking about a creative industry (books, film, tv, etc). In this context a synonym would be notion or idea (or maybe a metaphor).