Not sure if that's the right name... supposedly if you scan film directly on the glass you get optical fringes showing due to the very slightly differing air gap between different parts of the film and the glass (I realise that description goes beyond simplification into wrong, but it'll have to do until someone corrects me). You can get special glass film holders that are somehow treated to avoid the effect, and I think it's one of the reasons why high quality scans are wet mounted.
But if you're not seeing strange artefacts on your scans then I guess it's OK.