Assembly lines helps with the mass production of a product.
Monopoly of a product is important, because you control the majority of the item on the market.
If you had an assembly line producing say; blankets. You would be making lots of blankets. If the majority of blankets on the market are the ones that you made, then you have a Monopoly over them.
If you know that more supply=less demand, and less supply=more demand, then you know that prices fluctuate. If you were selling all of the blankets on the market, you could control the price of them to a certain extent, by holding some back, and making a greater demand.
That is Monopoly