I'd agree with Richard, that it means something that everybody wanted at that time. But it is confusing, because in the context of new technologies, you would expect IT (in capitals) to mean information technology.

Mmmm... it's true that IT stands for "information technology" but I think that maybe an "IT thing" means something that is hot and popular. Something is either "IT" and everybody wants it or it's "not IT" and people don't care about it.

"Information Technology" (computer science/programming)

"IT Thing" means something that is related to the information technology industry and field.