What it does
Melee is a combat keyword that rewards attacking multiple opponents. When a creature with melee attacks, it gets +1/+1 until end of turn for each opponent you attacked this combat. So if you swing at three different players, each of your melee creatures gets +3/+3.
The key nuance: the bonus counts the number of opponents you attacked, not the number of creatures or attackers you declared. It's calculated once, when attackers are declared, and locks in—attacking a planeswalker doesn't increase it, only attacking a player does. Also, the bonus applies even to a melee creature attacking just one of those opponents; every melee attacker gets the full count.
In Commander's multiplayer environment, melee shines because you naturally have three opponents. Spreading your attackers across the table can turn a modest board into a lethal swing, making it far stronger here than in 1v1 formats.
