A common enough skateboarding/snowboarding term. What it refers to is the long edge of your board that your heels are nearest to. The opposite side is toeside.
It can also be used to describe a type of turn used. Since turning in most board sports involves putting pressure on one edge of the board, a heelside turn is when you turn by putting pressure on the heelside of the board.
For instance: If a person snowboarding goofy were to do a frontside turn, the turn could also be described as a heelside turn.