They are called stunt pegs. You can stand on them, or let people stand on the 2 rear pegs to give them a lift. Also, well experienced bmx riders do "Grinding". This is where they have two pegs on the front and back wheel on the same side, and they grind on objects by jumping on to them and sliding across them. If you don't use them, you should take them off to save a lot of weight.
