For performing rotational tricks on flat ground, frontside means that for the first 90-180 degrees of the rotation, the skater can see ahead in the direction of travel. For example, a regular frontside 180 Ollie is rotated toward the heels or anti-clockwise (clockwise for goofy skaters). Conversely, a regular Backside 180 is rotated toward the toes or clockwise (anti-clockwise for goofy skaters). When riding fakie, a trick is considered frontside if it is done with the same motion that would be frontside if the person were facing forward to start.
For grinds, Frontside means that the skater is facing the rail/ledge during the approach. A Backside grind means that the skater is facing away from the rail/ledge during the approach.