"Bright" is when the treble of the sound is too harsh and it grates on your ears. Usually happens when the manafacturer "Cyrus" is involved and "very bright" happens when "Cyrus" and "Monitor Audio" are combined.
In use, if a particular hi-fi seperate is too bright (say amplifier), then another part of the chain should be "dark" ie, the treble is cut-off somewhat (therefore you would pick "dark" speakers). However, this is never a good approach to hi-fi and you should strive to purchase components which are "neutral" - ie, faithful to the original timbre.
edit: There's a rare occasion when things get sooooo bright a reviewer cannot convey the sound...this is usually referred to as "bright, bright, bright" and it happens when Cyrus, Monitor Audio, QED and pre-1995 CDs all come together for a hi-fi orgy!