An Assembly Line is a process where interchangable parts are added in a sequential manner to create a end product. First realized by Eli Whitney to create rifles for the U.S. Government, the system was used by Henry Ford to cut manufacturing costs and deliver a cheaper product.
Until the 1800s, craftsmen would create each part of a product individually, and assemble them, making changes in the parts so that they would fit together. With the ideas of the jaccard loom and a repeatable defined processes, it became possible to create parts in a repeatable manner. For larger products, the material would move on a conveyor belt or other conveyance and the workers stayed in place.
This linear assembly process or assembly line, allowed relatively unskilled laborers to add simple parts to a product, as all the parts were already made, they just had to be stuck in place. While there was still a requirement for the craftsmen to blueprint the design for mass production, they were no longer required for the actual assembly. While originally not of the quality found in hand made units, designs using an assembly line process required less knowlege from the assemblers, and therefor could be created for a lower cost.