The perceived wisodm is that optimists see their glass as half full, and pessimists see it as half empty. The point is, the vision is exactly same, it's only the interperetation that differs.
It's supposed to determine your outlook on life. If a person sees a glass as half empty they're deemed to be a negative person. If the glass is half full, they have a positive attitude, as in "Wow, this is great, I've still got half my wine left!" as opposed to, "Bummer, it's nearly all gone..."