Spells
Spells are the means through which you use and cast magic. Spells have an associated magic attribute, and an attunement slot cost. You can attune the same spell multiple times to increase the amount of available casts. Once you have learned a spell, you may attune to it during a Rest. At the end of a Rest, your available casts return to their maximum.
See:
Spell and Cantrip Attunement Slots
By default, characters "Learn" a spell as soon as they reach the requirements to attune to it, but a more hardcore campaign may require you to find a spell in writing, such as on a scroll, or within a tome.