Cooldown is one of the most fundamental concepts of Footmen Frenzy and Warcraft III in general. Cooldown is a term applied to spells and items. Cooldown is a delay that prevents heroes or units from repeatedly casting a spell or buying an item. Cooldown exists for balance purposes
Cooldown balances the game so that stronger spells have longer delays and weaker ones have shorter ones. If a Paladin could repeatedly cast Holy Light, then essentially, it could be invincible for the whole game. For example, if Hero A casts spell A, it must wait 10 seconds before being able to cast it again.
There are exceptions to cooldown, however. Abilities such as the Priest's Dispel, have no coolodown.