From DoomWiki.org

Melee is a form of an attack. In Doom, if the player runs out of ammo, he/she can use a melee weapons such as fists. Melee can be dangerous, but melee weaponary has a counterpart. Ranged monsters can cause severe damage to the player, unless he/she also has ranged weapons. Most enemies on Doom has melee weaponary such as bite. Using melee on zombies is safe, but Z-Secs or Commandos aren't safe to use melee on. The reccomended melee weapon is the flashlight, as it can do double the damage of fists.