Staves have charges which can be recharged when you invest the staff during daily preparation. And you expend charges based on the level of spell you are casting with it.
So for a staff with 4 charges, you can cast per day:
1 4th level spell
2 2nd level spells
1 3rd level spell and 1 1st level spell
or 4 1st level spells
Limited to the spells listed for that staff and lesser versions of that staff.
Each spell also costs 1 Resonance Point
It's not like a wand that gets used up and is gone. But it's not unlimited magic, either. Just a daily bonus to your spellcasting.