- 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 how the level of the unit specified compares with the level you set. With Relative ticked, the level you set is an offset from your own level, so -3 means three levels below yours.
Why you might use it
Compare a unit's level with a fixed number, or, with Relative checked, with your own level plus the value. While leveling, use it to skip long cooldowns and consumables on targets far below you, or to play more carefully against targets above your level. See also Level and Unit Classification.
Example
your target's level is less than 10
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.
Relative - When checked, Level is an offset from your own level (for example -3 means three levels below yours); otherwise it is an absolute level.
Operator - How to compare Level with the Level value. This comparison uses basic mathematical comparison operations.
Level - The level Unit's level is compared with.