Fake is a word that can be used as a noun, verb, or adjective in English.
As an adjective: fake means not genuine or counterfeit.
As a noun: fake refers to something that is not what it purports to be.
As a verb: to fake means to make something seem real or true in order to deceive someone.
In internet slang, fake is often used to describe people who pretend to be someone they're not.