Guarding the border for israel normally with a stolen buffer zone in Lebanon under the guise of keeping both parties out. In reality it means Hezbollah has to operate stealthily close to the border.
But they're rather inconvenient now that israel wants to invade Lebanon.
So does that mean this UN mission has failed then? A quick look suggests they are supposed to be keeping Hezbollah out of the picture and Israeli forces out of Lebanon. All of it wasted, there are over 10000 UN personnel there. Since they first went there in 1978 it seems they've been as much use as a sock on a fish.