Each spell sits in a Circle from I (apprentice) to IX (archmage). Higher circles hit harder but demand more casting minutes, the in-duel clock that limits how much you can do in a turn. A 1st-Circle bolt is near-instant; a 7th-Circle colossus takes real time to bring into the world, leaving you exposed while you cast it.
Balancing fast cheap spells against slow devastating ones is the heart of ArcoMage's tempo. When you sideboard between games you swap spells within the same Circle, so a spell's Circle also defines what you can trade it for. See Sideboarding and Best-of-Three.