You roll by holding the left analog stick to the right or left, and pressing jump at the same time.
You can only roll from side-to-side, not forwards or backwards.
They put in the roll because most MP shooting games become a jumping fest. Jumping will get you killed in SWBF. Rolling is the best evasion tactic.
Rolling is much faster than running, so you can cross the battlefield quickly by repeatedly rolling. The only problem is that you will not be able to see in the direction you are going.