Prowl is an alternative casting cost found on a handful of cards from the Shadowmoor set (mostly Rogues and Faeries). You may pay a card's prowl cost instead of its mana cost if, this turn, one of your creatures dealt combat damage to a player and that creature was a Rogue or Faerie (depending on the prowl ability's specified type).
The most common misunderstanding: the trigger requires combat damage to a player, not to a planeswalker or another creature, and pinging with abilities doesn't count. The dealing creature just needs to share the listed type—you don't need to deal damage with the prowl card itself.
In Commander, prowl is a fringe mechanic. It shines in dedicated Rogue/Faerie tribal decks built around evasive attackers, letting you chain cheap spells after connecting—but it's situational and rarely a primary strategy.