What it does
I'm not aware of "Exile Cannon" as an official Magic: The Gathering keyword or ability. It doesn't correspond to any recognized rules term, keyword action, or ability word in the game.
It's possible you're thinking of something else—perhaps a specific card name, a custom/homebrew card, or a casual nickname for a strategy (like a deck built around repeatedly exiling permanents). "Exile" itself is a game zone where cards are set aside, often permanently, and many effects "exile" cards as a more powerful form of removal since exiled permanents typically can't be recurred from the graveyard.
If you can share the exact card or where you encountered this term, I can give you an accurate explanation of how it works and why it matters in Commander.
Iconic cards with Exile Cannon
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