In the "no docs are bad docs" department today, go to gulpjs.com
and look at the page. Scroll down so you can read it all, then the Docs link and/or the "Try it now" link (they go to the same place). If you want, you can also follow the "Plugins" link. The one other link on the page, "Twitter", I didn't bother following.
After all that, tell me, in simple terms, what does it do?