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