The magnitude is the brightness of an object. To give you an idea of what different magnitudes are, read this:
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The Sun shines at about mag. -26!
The Full Moon shines at about mag. -12.
The brightest Iridium Flares reach mag. -8.
Venus, when at it's brightest, is at about mag. -4.
Sirius, or the Dog Star, which is the brightest star in the sky shines at about mag. -1.4.
In a dark sky, the naked eye can see down to about mag. +5.
Pluto shines at a faint mag. +14!
But that is bright compared to what the Hubble Space Telescope (HST) can see! The HST can see down to mag. +30!!!
The reason the magnitudes go into the minus's, is because hundreds of years ago, the astronomers made a system for the brightness of every star in the sky, with mag. +5 being the faintest then could see with the naked eye and mag. 0 being the brightest. Now a days, some of the stars in our sky are brighter than the ones back then. So the astronomers had to "update" the system with the magnitudes of the brighter stars. So instead of changing the whole system around, they just went into the minus's.