Film yeast is yeast that grows on the surface of cider. It isn't like normal yeast because it needs oxygen, that's why it stays on the surface. It is the floating whitish stuff. Small amounts aren't really a problem except they look terrible. If you have a floating film of yeast or bacteria on your cider it means air is getting in, so you are probably doing something wrong like a bad seal. If you use a carboy filled near the top there isn't much space for a film. If you bottle still cider and air gets in you may get a bit of film yeast, makes the cider unsaleable but if it is just homebrew it is more of an embarrassment than anything else.
If you do things right film yeast is just something you read about on forums.