A "Stunned" creature can’t take actions and is considered helpless. Generally for 6 seconds if from the stunning blow feat, but the duration will vary if from other sources. Note the stun caused by the spell Symbol of Stunning doesn't really induce the conditioned stunned, it's actually considered dazed, as you aren't rendered helpless.
List of abilities/spells that inflict the stunned condition:
- Tomb of Jade (against Tainted creatures)
- Jade Strike (against Tainted creatures)
- Stunning Shield
- Shield Champion