What it does
I'm not familiar with "Brave Heart" as an official Magic: The Gathering keyword or ability. It doesn't appear in the comprehensive rules or on any printed cards that I can verify.
You may be thinking of a few possibilities: a custom or playtest card, a misremembered name, or perhaps "Bravado" or "Boast" (an ability where you pay a cost when a creature attacks for a bonus effect), or even "Heroic" (which triggers when you target a creature with a spell). There's also "Valiant," a newer ability word that triggers the first time each turn a creature becomes the target of a spell or ability you control.
If you can tell me the exact card or set where you saw "Brave Heart," I can give you a precise, accurate explanation of how it works and why it matters in Commander.
Iconic cards with Brave Heart
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