U Jelly is a phrase/meme used on forums, most commonly 4chan, to ask a person if they are jealous. It's usually accompanied by a picture of a guy with a smug smirk, with 'U Jelly?" Over his head.
It's used to ask forum goers are they jealous, because someone on
the forum has
something that others don't have, and they want to show it off, to make
other people Jelly.