- 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 the game reports the unit specified as friendly to you. Being friendly does not mean the unit is in your party or raid.
Why you might use it
Check that a unit is friendly to you before suggesting a heal, buff or dispel on it. Typical uses are mouseover healing (only offer Flash Heal on a friendly mouseover target) and spells that change with the target, such as Holy Shock healing a friend instead of damaging an enemy. Friendly does not mean grouped: a friendly player outside your party matches too, so use In Group when membership matters. See also Hostile.
Example
your mouseover target is friendly to you
Settings
What each setting in the condition editor means, from the game's own help.
Unit - The unit this condition uses as the target of comparison.
Matches when the game reports the selected unit as friendly to you. This does not imply party or raid membership. A missing or unreadable unit is unavailable.