Snow is a base terrain, found only in polar regions (extreme north/south of the map, past Tundra). It has no base yield, and usually no other uses.
Strategy
Snow is by far the worst possible terrain to settle near. It provides no benefits at all - no Woods, no Rivers, and only a few resources can be found there - though it may still have Lakes or coastlines. That said, some late-game strategic resources may be found in the Snow, sometimes making polar colonies worthwhile. Try to settle on Snow tiles only along the Coast, or in close proximity with other terrain types - otherwise, any city founded there will have neither 
Food nor 
Production, and you will be forced to develop it via 
Trade Route Trade Routes only.
As a general rule, you should consider settling on a Snow tile only if you are playing as Sweden, trying to grab some rare 
Uranium that you do not already have in your empire, or planning to build the Amundsen-Scott Research Station.
Civilopedia entry
History offers very few examples of human survival in snow-covered lands in cold climates. With a few exceptions, such as the Inupiat, humans have had a hard time making a living in the far polar regions.
Gallery
.png "Snow tile in-game (Civ6).png (617 KB)")