What it does
Cheer isn't a real Magic keyword, so there's nothing official to explain here—you may be thinking of a different ability or a fan-made/custom card mechanic.
A few possibilities you might mean: Cleave (from Crimson Vow), which lets you pay a higher cost to ignore bracketed text on an instant or sorcery; Cycling, which lets you discard a card to draw; or perhaps a casual playgroup's homebrew term. If "Cheer" comes from a specific set, card, or custom format, the rules would be defined on that card or in that format's documentation rather than the comprehensive rules.
If you can point me to the exact card or source where you saw "Cheer," I'll gladly break down how it works and how it plays in Commander.
Iconic cards with Cheer
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