Silver is one of the base metals added by Thermal Foundation. It can be obtained from Silver Ore and, less commonly, Lead Ore.
Silver Ore and Lead Ore can also be processed in various ways in an Induction Smelter, with varying byproducts.
Like most base metals in the mod, Silver can be used to craft tools and armor. It also has various uses as a generic crafting ingredient in Thermal Expansion. More importantly, Silver is a component in various alloys: Electrum, Signalum, Lumium and Enderium all require different amounts of Silver to create.