The AR15 gas block is a part of gas system that replaces the standard front-sight base.
On the AR, there is a small “gas port” (small hole) in the barrel that vents gas with every shot. This gas travels through the gas block and gas tube into the receiver, where it powers the bolt carrier group and auto-cycles the next round.
A low-profile is a smaller gas block that is made to fit under a free float quad rail. There are also "railed" gas blocks that allow you to mount an iron sight on the gas block.