An auto is where you land the heli without any assistence from the engine. There are three reason why you do this.
1. Your engine stops midflight because of failure or you were just an idiot and forgot to put fuel in it.
2. You like the rush of flicking the throtle hold switch on your transmitter and landing the heli on its own
3. You like to practise the above technique so as if number 1 happens you dont ***** ya pants.

To get it to autorotate you pretty much set your heli up so as when you flick the throtle hold swich (on ya TX) and move the the throtle stick down it puts the main blades into a negitve pitch. This enables the blades to gain/retain headspeed from the air going thru them as the heli hurdles towards earth. As you get about 10 feet off the ground you give the Tx some collective (this turn the blades into a positive pitch) which causes the heli to slow down and then hopefully into a hover at around 2 feet at which stage you can lower the collective a little causing the heli to slowly land. Sounds easy but there's a lot of skill to make it work. There are are couple of other things you can do to make it easier but this gives you the general idea.