So you pick 2 domains. Each domain allows you to cast specific spells. This is where clerics can get interesting because you can gain access to spells not on the cleric spell list
So let's say you pick the air domain. Starting out, you get access to the lightning arc power. This is a magical ability, but it does not use spell slots. Instead, you get to use it 3 times per day, plus another 3 due to your wisdom modifier to a total of 6.
Maybe I haven't explained this right. A cleric is a prepared caster. That means at the beginning of the day, a cleric gets to pick any spells on their spell list that they would like to cast that day, but after they pick them they're stuck with for the day. But a cleric also has the class feature "spontaneous casting", which allows them to swap out any spell they currently have prepared for a healing spell.