Also keep in mind it is not a compliment to be pretentious It is a critical term

It’s about “trying to be smart” or “attempting to act either know of things which one don’t know or other way around”. If you know the meaning of “pretend” then it’s easy to understand. Hope it helps.

Hi, here is a defenition for the word according to Google: attempting to impress by affecting greater importance or merit than is actually possessed. So basically it is an adjective that is related to a verb which is "to pretend". So, for instance, a pretentious person is a person that pretends to be someone or something that the preson is not.