Cogs are the teeth on gears. Metaphorically, they are used to refer to one small part of a machine, which are not useful on their own, but in aggregate can get

Cogs are the teeth on gears. Metaphorically, they are used to refer to one small part of a machine, which are not useful on their own, but in aggregate can get useful work done. The term is sometimes applied in a derogatory fashion to people, implying that they are nothing more than a replaceable cog in a large machine.