Mass-produced things are made on a large scale, usually in a factory or plant. Cars, computers, and sneakers are all mass-produced. Your dad's famous chocolate chip cookies are not.
Mass-produced items are standardized, if not identical to each other. A factory that makes toasters might produce tens of thousands of toasters that look exactly the same. They're made in such huge quantities on an assembly line — they're mass-produced. The process itself is called mass production. These terms were first used during the growth of the automobile industry in the 1920s.