It means the moon is between around #120 and 150^@# from the sun.
Explanation:
At #90^@# from the sun, the moon is waxing and half lit, and at #180^@# we see a full moon. The word gibbous is used to describe a moon that is more than half, but not fully lit. There is thus a waxing and a waning gibbous moon in each lunar month. A diagram should help make it clearer: