An Enchantment is a property of powerful magical Weapons and Armour that increases their base effectiveness in battle. Enchantment values range from +1 to +5.
An item with an enchantment value is always at least of Uncommon rarity, and possibly higher. Some magical weapons and armour pieces - particularly those with names and histories - have an enchantment value in addition to a variety of possible item-specific properties.
Weapon Enchantments
Weapon Enchantments add a bonus to Attack Rolls and Damage Rolls equal to the enchantment value.
For example, a mundane Shortsword deals a base damage of 1d6 (1~6)
Piercing. A Shortsword +1 increases that base damage to 1d6+1 (2~7)Piercing.
Armour Enchantments
Armour Enchantments increase the base 
Armour Class of armour by an amount equal to the enchantment value.
For example, a mundane Leather Armour has a base Armour Class of 11. A Leather Armour +1 increases that base Armour Class to 12.