What it does
I'm not aware of "Starfall" as an official Magic: The Gathering keyword or ability. It doesn't appear in the comprehensive rules or on any printed cards I can verify.
It's possible you're thinking of a custom or unofficial card (Mythicwyrm may host homebrew content), a card name rather than a keyword, or a similarly-named ability from another game. There are official cards with "star" in the name, and mechanics like "Constellation" (triggers when an enchantment enters) from Theros, but nothing called "Starfall."
If you can share the exact card text or where you saw it, I can explain how it works and its Commander implications. For homebrew abilities, the reminder text on the card itself is your best authority, since it won't be covered by official rulings.
Iconic cards with Starfall
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