Stun is a status effect that prevents active actions like moving, turning, using spells or items, but does not prevent passive abilities. Stuns also interrupt spell casts and channeling spells.
Many spells cause stuns, which makes them effective for stopping and killing enemy heroes. Stuns also allow for easier escapes as they temporarily prevent the enemy from pursuing. It is usually considered to be the strongest disable, which is why most stuns do not last for more than a few seconds.