English: The mainspring barrel from a clock. The mainspring which drives the clock, a coiled ribbon of spring steel, is inside the barrel. The clock is wound up by fitting a key onto the squared-off axle of the barrel (visible) and turning it, winding the spring inside closer around the axle, storing energy in the spring. The other end of the spring is attached to the barrel. The force of the wound up spring turns the barrel, and the ring of gear teeth around the outside turns the clock's gears, until the spring runs down and needs to be wound again.
WARNING: Do not open the device you are seeing on the picture unless you have the special tools and training needed to perform this work. Mainsprings contain a lot of energy. Even a broken spring is powerful enough to injure you or others very seriously! Do not disassemble clockworks by yourself, unless you know how take the necessary precautions. Disassembly of a clockwork without such precautions may cause serious injury of you or other persons. You may also destroy the clock. These pictures are not presented here to inspire you for disassembly, but to give you the insights without having to disassemble a clockwork by yourself!