After a while your disc brakes will need adjustment. If you have not had previous experience adjusting disc brakes don't worry, it is not hard to do!
How Disc Brakes Work
On the inside of the rotor: you will have a brake pad that does not move. You can adjust the position of this pad in and out until it is as close to the rotor as possible without touching it using the Allen bolt on the inside of the brake.
On the outside of the rotor: you will find a brake pad that moves in and out with the brake lever. This pad squeezes the rotor against the stationary pad which causes friction and the bike to slow down. You also want this pad close to the rotor without touching it. To do this turn the barrel adjuster counter clockwise to bring the pad closer to the rotor.
That is it! Now that you understand how disc brakes work, please see this video for a more detailed explanation that will make more sense...