[@Dynamo Frokane]

Basically when you rest at a Bonfire (the checkpoints... kinda) you can attune a spell. Each spell can be cast only a certain number of times. After you cast it that many times you have to rest at a Bonfire again to renew the charges.

The comparison is that the cards are like the spells, and in one fight you may use any given card a certain number of times before it runs out of 'charges'.