Lever on right side of engine on my yamaha atv 250cc engine?

on the right side of my yamaha engine opposite fuel lever side theres a lever that says on that is on the cylinder, all it says is (0n) you push it up. what is this lever? also have brake issues wanting to find a easy way to fix,how?|||hmm...it may be a ''decompresion valve'' used to aid starting..|||Is it a reverse lever? I have seen those on some of the smaller atv's. Also, more information is needed for the brakes. Just saying you have issues is not enough.