One can think of cooldown as the reload time and firing rate of weapons. For example, a machine gun has very fast firing rate, so it has a very low cooldown between shots. Comparatively, a shotgun has a long reload/cooldown time between each shots.
In design terms, cooldown can also be thought of as an inverted 'casting time' where instead of requiring a wait time before using an ability, cooldown may replace casting time and put the wait after the ability is activated. This creates a new dimension to the balancing act of casting speed versus power: "lower cooldown, faster cast, but weaker strength" versus "higher cooldown, slower cast, but greater strength."
From the technical point of view, cooldown can also be used to assert control over frequency of cast (for spamming) in order to maintain a fluid frame rate and ping. For example, in the game Diablo II, cooldown was added in the form of a patch to several graphically and CPU intensive spells (blizzard, frozen orb, hydra, etc.) to solve the problem of extreme lag caused by players spamming these spells in multiplayer.