You probably already know the meaning of in as a preposition. It’s one of the first things you probably learned in your English class e.g. the boy’s in the house, my pencil is in my pencil case. But it can be used to mean something completely different—it means to be in fashion or trending at the moment. Things that are in at the moment may not be in in a month—why? Because trends always change!
“Jordan, why do you keep listening to that music? It’s awful!”
“Mom, you don’t know anything. It’s totally in right now!”
“So, what’s in at the moment?”
“Yeah, come on I wanna know what’s cool and what’s not!”