The word magnitude describes the brightness of a celestial body and the value of magnitude is inverse proportional to the degree of brightness.
Take as example the magnitude of 1 of a star and the magnitude of 10 of other star. Maybe you are tempted in judging the biggest value as an indicator of a higher degree of brightness, but it is not at all the case. The body having the magnitude of one unit will be brighter than the body having the magnitude of 10.
So, keep in mind, the smaller the magnitude is, the brighter is the body.
If you see a negative value of magnitude, you should know that is not a mistake but the measure of a very high degree of brightness.
Answer: Magnitude describes the brightness of celestial bodies.