When something is popular or well-known, you can call it "a thing". For example, you can ask someone about a phrase that they used which you've never heard before:
A: His ideas are so out-of-the-box. They're boxless!
B: "Boxless?" Is that a thing?
A: No, I just made it up.
Or you can use this phrase to talk about something that's fashionable:
Huge, baggy suits were kind of a thing back in those days.
Using "a thing" in this way is pretty new and more common with younger people. It's a casual expression which you wouldn't use when writing an essay or giving a formal speech.