What is an ITopic? I've seen many users refer to it and I have no idea what it is…

I will say thank you in advance

An ITopic is this.

ITopics are threads with useful info. They're used in forums with lots of stickied threads, like the Questions forum. The Requests forum has very few stickies, so it doesn't really need ITopics (therefore there are no ITopics in that forum.)
An ITopic means “Index Topic”, which makes sense considering it's similar to a sticky but isn't actually a sticky.
Usually a sticky will have links to ITopics, or vice versa.