Question about my recently bought lens.

So basically I have a Sony 70-300mm g ssm and it just came today(still figuring out how to use it) and for the auto focused there are three switches, MF, FULL infinite sign-3m, and AF. I'm not sure what the second button is and how I use it properly. Also on the spot where it says metres and feet there is also some infinite sign at the end and I'm confused to what these do.also, last thing, there is some button near the end of the lens that does nothing? I'm not sure what it does.