Magnitude is a measure of brightness of an object. The brighter the object the lower the magnitude number. The generally accepted value for the Sun is -26 while the accepted magnitude limit of human vision is magnitude 6. The Hubble space telescope can see down to a magnitude of 30. My deepest detected star was at a magnitude of 24.0 in a stack of ten 300 second exposures. However, nights with conditions that good are very rare.
The scale is logarithmic with a difference in brightness of 2.512 between magnitudes. A difference of 5 magnitudes is 100 times brighter.
There are two classifications of magnitude:
Apparent magnitude – this is the magnitude as seen by an observer on the Earth. The Sun is the brightest star we can see only due to its proximity. There are many stars brighter than our Sun but have a dimmer magnitude because they are so very far away.
Absolute magnitude – this is the observed magnitude if the object was moved to a standard distance away. For stars that distance is 10 parsecs. For solar system objects the distance is one AU from the observer and one AU from the sun. The distance for meteors is 100 Km on the observer’s zenith.