In skateboarding or other roller sports, this term basically means doing an ollie (or jump) up onto the edge of objects like tables, rails, fun boxes, etc. with a skateboard and grinding it along the surface until you pop off of it back onto another surface or back onto the ground.
A dance performed (often in a club setting) in which the "grinder" places his hands on his head with his feet planted in the ground and proceeds to swivel vigorously
When you work really hard and put a lot of time towards something. Used commonly in the context of achieving a goal or object of desire, whether that be in real life, a sport, a video game, etc.
Jeff: "Nice car dude, how could you afford that?"
Mike: "Man I just been grinding for it, been working hard at it. Spent a lot of time trying to get this thing."