I'm not exactly sure, but I think it's kinda like a primer. I don't use a primer, really, since I don't use eyeshadow all that much. When I do, I just powder my lids and then apply. Mineral foundation does the trick for the most part for me. However, when I want to wear my eyeshadow for an extended amount of time (more than 8 hours), I need something else. Any recommendations?
Are you referring to a "mixing medium" for use with loose mineral eyeshadows?
Loose mineral e/s can be used dry or wet. By using them wet you get a more intense color payoff and for some people it makes the e/s last longer. MAC sells a mixing medium but I've never used it. Some ladies use plain water, Visine eye drops, Milan Gelle drops, Ben Bey Liquiset, various brands of eyeliner sealant.