Slit on roof of mouth

It's where your upper jaw (pallet), if you will, comes together. Before I had braces back in high school, they had to break my pallet in order to get a stray tooth to come in correctly. I wore what was called an expander. There was a platform in the middle with four legs that were cemented to four molars. In the middle was a hole for the tool I used which was cranked only twice per night. Over the span of about 2 months it split my pallet, let my stray tooth come in...and left a temporary split in my two front teeth.

Anyways, that is what that slit is. Unless your mouth is growing I doubt there is anything to worry about.