OK, apologies if I've dumbed it down to an insulting level, trying to avoid too much jargon...
When you have an extraction done, it makes quite a big hole in your mouth. (No sh*t Sherlock!) As a result your body pours reconstructive cells into the area to, well, mend it
There will be little areas like this where some of these cells congregate in higher numbers. It's part of the normal healing process. If you sliced one of them open and had a look under a microscope (DON'T ACTUALLY DO THIS) then you'd find it was full of epithelial cells, some bone generating cells and lots of white blood cells (to munch away superfluous bits) to make room for the new bits to put it all back together again.
If you keep watching, you'll see the little little lumps and bumps come and go, it's actually a fairly interesting process (if you're a sad geek like me anyway).
The hole will gradually fill up from the bottom, you can see the highly specialised granulation tissue already forming in the hole, (the sort of greyish/yellow stuff), and then epithelial cells will spread over the top from the edges to seal it all up.
The posh name for it is "Healing by secondary intention" you can find out more details from Google if you're interested.