Production lines seem like something that sprang up at the turn of the 19th century, but they've actually been around for longer than that. Basically, a production line uses a division of labor. In a division of labor system, instead of each person doing an entire job from start to finish by themselves, everyone takes a small piece of the job, bringing the smaller parts together until the work is done.
Of course, people have always divided up work around the house or farm. But for most of human history, if something had to be built or crafted, one person would make it from start to finish. Skilled craftspeople perfected the art of making one particular product. They'd train others to do the same work, taking a product from start to finish. Once the product was complete, the craftsman could then trade the finished product for other goods that he or she needed.
The only problem with this system is that it's very time consuming. In addition, it may take several years of training to become a skilled craftsperson. It also made the goods that craftspeople made very expensive.
When craftspeople began parsing out the individual tasks involved in building a specific product, they found that the work went a little faster. Initially, people were still doing semi-skilled labor in their own homes. For example, a milliner might have one person at home cutting hat patterns, while another assembled fake flowers, another tied ribbons into bows and another sewed the fabric together, and finally another person assembled the finished hat.
Still, those jobs required a little skill, and the process was a bit on the slow side. With a more mechanized process, however, people soon discovered the process could move a lot faster. Also, with more machinery involved, the people making the product could be less skilled. For instance, instead of having to find a worker who knew how to cut hat fabric to fit a pattern, a milliner now only needed to find a person who could load fabric into a cutting machine. Instead of knowing how to sew, workers simply had to run fabric through a sewing machine. The process was faster, and because the labor was unskilled, it was cheaper, too. The shift to add machines into the production process made it possible to mass produce a variety of products -- including cars.