From WoWWiki

Usable in the Main Hand or Off Hand, as opposed to Two-Handed weapons.

Weapons or Items identified as Main Hand are also One-handed, but can only be equipped in the Main Hand allowing an additional Weapon or Item to be equipped in the Off-Hand.

Additional notes:

• If the character only has one weapon, it will Equip in the Main Hand Slot first.

Equipment and Weapons
Armor	Cloth • Leather • Mail • Plate  –  Cloak • Necklace • Ring • Trinket • Shield
Armor sets	Druid • Hunter • Mage • Paladin • Priest • Rogue • Shaman • Warlock • Warrior
Melee weapons	Axe • Dagger • Fist • Mace • Polearm • Staff · Sword One-handed • Two-handed • Main hand • Off hand • Dual wield
Ranged weapons	Bow • Crossbow • Gun • Thrown • Wand  –  Arrow (Quiver) • Bullet (Ammo Pouch)
Relics	Idol • Libram • Totem • Spellstone/Firestone