Silk is one of 31 rare resources. It appears as a collection of large frames draped with some sort of fabric with spiral patterns and some (white mulberry) plants nearby, both of which being used by silkworms to produce silk as well as another frame draped with colorful fabrics, likely the processed silk.
Like all rare resources on land, Silk can be harvested by sending a Merchant from the Market to the resource location. Silk provides the following benefits:
The gather rate of rare resources can be increased in various ways, as described in the main article.
Silk is one of the five rare resources to reduce a research cost, with the other four being Furs, Dye, Papyrus and Silver. The cost reduction is quite significant, so if Silk happens to be available nearby, it should be hooked up with a Merchant as soon as possible.