What it does
I'm not familiar with a Magic: The Gathering keyword or ability called "Caan." It's possible this is a misspelling, a fan-made term, or something from outside official Magic rules.
You might be thinking of similar-sounding mechanics like Cascade (cast a spell, exile cards until you hit a cheaper nonland card, then cast it free) or Convoke (tap creatures to help pay a spell's cost). Both are popular and impactful in Commander.
If you can confirm the correct spelling or share the card text you saw, I'll gladly give you a precise breakdown of what it does, the rules nuances people get wrong, and how it plays out in EDH.
