It describes a quality or characteristic, such as:
* A humble person: Someone who is modest and unassuming.
* A humble beginning: A simple or insignificant start.
While "humble" can sometimes function as a noun, it's not a common usage. If used as a noun, it would refer to the state of being humble, such as:
* He showed great humility.
* Her actions were a testament to her humble nature.
In these cases, "humility" would be the more common noun form.