Melee is close quarter physical combat.
In most roguelikes bumping into an enemy with the normal movement interface causes a melee attack.
Melee attacks are typically not guaranteed to hit. The specific mechanics of such combat vary greatly from game to game and these differences are frequently a fundamental feature of the gameplay experience.
Most roguelikes feature a wielded weapon or two which will be used automatically during attacks. The worn armor and shield of the target will affect the outcome.
Some common considerations when designing a melee system include