Mixing medium is basically paint without any colour in it, and it can be used for thining paints or inks without them going watery. You can use it to thin ink to make glazes that don't run into puddles or dry with tide marks.
It's great for dirty washes - try taking a drop of brown ink (preferably the old GW brown if you have any left. PPs is too red), a drop of Battlefield Brown paint, a few drops of the PP Armour wash and loads of mixing medium, then slapping this over a basecoat of Pig Iron. It looks horrible at first, because the mixing medium is milky when wet, but when it's dry it makes a superb dirty iron effect. I mixed a whole bottle of the stuff and it's my standard wash for any metalwork on Warjacks or Space Marines. I can't claim credit for the recipe though, as it's Quentin Smith's from NQ20.
Like any washes, don't mess about with it as it's drying.
All the metalwork on both the models shown have a wash of the mix described over a plain basecoat, and very little other shading.