Averted vision is when you look slightly to the side of something as opposed to looking directly at it. Eg the moon (not that this is a good example but its a easy one) if you look directly at the moon it is in the center of your field of view. However if you look to the left (or any other direction of it) it still in your field of view but your vision is averted from it. This is a good skill to master when looking for dso's and once u know what ur looking for its handy for finding random objects.
Some of the best unplanned observing I have done is locating something by averting my vision and then pointing the scope in that direction to see whats there
the fun part is usually trying to work out what the hell I was looking at!
Daniel