Autorotation is probably the best thing out there. It ranks above fire, electricity, and sliced bread hands down.

Like mrasmm says, it's when the engine dies on you (or is shut off purposefully when training) but you're still able to land safely. Basically, during decent, you change the pitch of your blades so that they catch the air and start turning. The purpose of this is to get enough blade rotation so that at a certain distance above the ground you change your blades' pitch again so that now they're creating some lift for you (this is called a flair) and slows your plummet down to safe landing speeds. Again, like mrasmm suggests, YouTube has some great examples of this, both real and R/C helis.