Backside refers to rotations or turns done so that the skater's back is facing the outside of the turn arc. For example, if you are regular foot, a backside turn would be a right turn, with your front facing to the inside of the turn. If you are goofy foot, a backside turn would be a left turn, with your front facing the inside of the turn.
The name Backside came originally from surfing, as did a lot of skateboarding's original language. The first skaters were in fact sidewalk surfers.
The opposite of backside is called frontside.