Hiltslashing in general could mean hitting very early in the animation by looking next to your opponent when you swing, pretty much hitting them with the hilt. However I think most people refer to a specific left-right swing combination.
It's done when facehugging and moving into your opponent. You do a leftswing, they block it, then you do a right swing while moving your camera with the swing. If you do the camera movement and footwork properly you'll do a second attack before your opponent barely starts his attack. It's basically just a very fast follow up attack, due to hitting your opponent very early in the animation, doing damage with the hilt. Its possible with both polearms and 2h. Think its pretty much the same doing it with both classes, its only the speed rating that makes it easier or harder.
They tried making hiltslashing impossible by adding a slight delay to do a second attack after your swing gets blocked, but its still possible. I was able to do it quite well, somehow couldn't do it anymore later on. After this change I can only do it with katanas and longswords. Like Dezilagel said, the amount of players that can pull this off confidently and properly are quite limited.
