Combos are for hitting lanes or orbits with little or no pause. You actually have about 5 or so seconds after hitting a lane or orbit to hit another one. BTW lanes are in the middle of the table, and an orbit must go all the way around. The orbits are harder to complete because they take longer and so you have less time to hit another lane.
Also it says combo in front of each lane. As long as it's lit up, your combo is still active. If the lights go out, you must start again.
This takes a few tries to get the hang of it, but for this game. it's not too hard.
Good Luck.