When you convert from a high bit depth, like 24 bit, to a lower one, like CD-standard 16, there’s going to be a loss of quality and an addition of distortion.  It’s not a lot, but it can be some.  What dithering does is it adds a special noise of its own, usually at frequencies above the range of human hearing, that (approximately) compensates for the distortion added by changing bit depth.  Darkroom mastering has been kind enough to write up an excellent detailed explanation.  Generally, dithering is not something you have to worry about when mixing, and only the mastering engineer needs to care.