In skateboarding it is a way to maximise the amount of points you get in a competition, or just to show off. Combos are a string of tricks linked together, so if you start with a kickflip and end with a backflip (doubtful) then all tricks inbetween are included as one trick, and are multiplied.
ex. If you do a kickflip in a competition on its own it is worthless. If you do a kickflip and a pop shove it they add their stand alone worth and then multiply it by 2 for the difficulty. This is just an example, it wouldn't happen exactly like this in real life.

Flatland Techers, such as Rodney Mullen, string together long combos using manuals, Kaspers and flips. It can also be used in street to string together grinds.