Resin is a type of plastic. It is extremely durable for harsh weather. You can leave it outdoors year round, however if left under snow you might see pollution effects on your furniture. Leaving your resin furniture under snow will not damage its structure but might weaken its finish.
Pollution in snow might leave marks on your resin furniture which will be very hard to clean. To avoid this it's best to cover them. However if you have resin wicker furniture that has a different set of rules for snow since the frame could be made from various materials such as aluminum, iron or wood. We recommend covering in most cases.