A common acronym found in internet forums. It means In This Thread. It's used by lazy posters who want to communicate what they are looking for with a minimum of words. Typical usage might be something like "ITT pictures of kittens", signaling that the poster wants other people to post pictures of kittens.
Not at all appropriate on E2, or even on most forums, as it is a extreme case of non-communication on the part of the poster. No explanation of why they want kittens, no commentary on the social worth of kittens, simply a brief and forward request.