Why is it filmy!

[Aimee 911]

Hi Everyone,
I've been drinking wine for years, but have realized I really don't know much certain things, like why does my wine glass get a filmy coating around it when I swirl the wine around? Does that mean that the wine is not good? It seems like this usually happens when I buy a wine on 'mega-sale'

You're expertise will be much appreciated!!

Thanks and happy tasting
Aimee