Despite the name, German silver is actually an alloy of copper, nickel and zinc. Sometimes called nickel silver, the alloy has properties ideal for watch movements, being non-magnetic, corrosion-resistant and hard, but easy to machine and polish.
It’s typically used for the largest parts of the movement such as the bridges and base plate, and is most closely associated with German watchmakers like A. Lange & Sohne, which uses the metal for all of its movements, such as the calibre L121.1 that powers the Lange 1.