It basically means 'You jealous?'
Hey guys, I just got the new HTC Droid! ujelly?
A contraction of "you're jealous". Popularised in late 2010 alongside a macro of a man grinning. Sometimes written in greentext as "you jelly".
Person A: I've got a mosin nagant!
Person B: FUCK YOU MOSINS SUCK
Person A: ujelly?