- Not available on that edition (the reason is in the tooltip)
- Depends on the setting, your class or another addon (details in the tooltip)
What it does
Checks if you are currently in the shapeshift form or stance you choose, matching any rank of it. Choosing humanoid checks that you are not shapeshifted at all.
Why you might use it
Check your shapeshift form, which also covers Warrior stances and Stealth: a Feral Druid keeps Cat Form abilities out of the Bear Form part of a rotation, a Warrior suggests Overpower only in Battle Stance, and a Rogue opens with Cheap Shot or Ambush only from Stealth. Choose humanoid to mean not shapeshifted, for example as a switch condition that gives a Druid a caster rotation. On Retail and Forever, if the game hides your active form, the condition is unavailable (see also Stealth).
Example
you are in humanoid form
Settings
What each setting in the condition editor means, from the game's own help.
Form - You are shapeshifted into a different form. This includes stances for warriors and stealth. humanoid can be used to indicate you are NOT shapeshifted.
A form is chosen by its spell, so the condition keeps working as new forms are learned and on every client language. Any rank of the form matches. The list shows the forms on your stance bar, then your class's other forms on this client. A condition saved by stance bar slot keeps that slot until you choose a form. If the client cannot read the active form, the condition is unavailable, including under NOT.