Mass production is a way to make a large quantity of goods as efficiently as possible. Cars are assembled in factories using mass production.
When thousands (or millions) of copies of the same thing are produced, that's mass production. An auto factory is a great example of this: workers on an assembly line, aided by automated machines, manufacture one identical car after another. Many of the goods you use every day are mass-produced, from your toothbrush to your mobile phone. Mass means "a large quantity," though its root means "kneaded dough."