SMC usually refers to Super Mutii Coating when referring to filters and lenses.
Modern quality lenses have multiple coatings on the glass elements which give that colour you see when you look at the front element. These coatings are applied to all elements of the lens, even the ones which are cemented together. The cumulative effect of all these coatings is to reduce flare and light scatter (diffraction) within the lens, and a result, these lenses transmit significantly more light than uncoated lenses, and also produce far better colour fidelity and saturation.
Sticking a cheap filter in front of an SMC lens has to compromise image quality to an extent, which is why SMC filters are produced to a similar standard with mutiiple coatings. Inevitably, they're usually a lot more money.