A brilliant frequency spectrum analyser. It show you what frequencies the instrument or voice produced (x axis) and how much of them (y axis).
You plonk it on a track as an insert effect, after compressors, eq's etc. Turn all of them off, and you'll see just the bare track, turn them on and you'll see the difference they make.
If you put it on the master buss, that'll give you an idea of how the mix looks, it should be sort of flat a long the top, but I prefer a bit more a of a slight hump near the bass end and a tail off at high frequency end.
Pretty cool, but do spend a bit of time getting to know it, as with all plugins, practise is really time well spent.
You'll find it here