Stun is an affliction; a negative status effect that handicaps the target.
A creature under the effect of Stun is unable to move or take any action while they are stunned. They also can't dodge or parry any physical attacks directed at them while stunned.
Stun as spell
Players are able to inflict Stun on a target using the Stun spell from the Defensive Affliction Magic skill. This spell is first available at level 40 defensive magic. Like with all affliction spells, a link must be maintained to keep the target stunned.
Players can increase their chances of resisting a Stun Affliction spell by wearing jewelry that raises their Lakelands resistance.
Stun by critical
The Stun caused by a critical hit to the head is not a magical effect and thus Lakelands resistance will not help resist it. Creatures that are immune to Stun spells similarly are not immune to this effect.
Enemies using Stun
Since homin mages are capable of casting Stun affliction spells, it may be used against you in PvP. It may also be cast against you in PvE by bandits or tribes guards, who do not need to maintain a link to keep you stunned.
Kipees have an additional special ability that allows them to inflict Stun on their enemies with a headbut. This Stun lasts much longer than one caused by a critical hit and does not require the Kipee to maintain a link. It is also not a magical effect and thus able to bypass both Lakelands resistance and Stun immunity.
Creatures immune to Stun
Stun immunity only applies to the Defensive Affliction spell; not to Stun by critical hit or the Kipee special ability.