FDM/FFF 3D printing is a type of additive manufacturing, meaning material is added (instead of cut away) to create something. 3D printing extrudes hot, molten plastic in layers, one on top of another, to create an object.
It's much cheaper than injection molding for small production runs (1-10,000 parts). It's also a lot cheaper for small businesses like us to acquire equipment, letting us pass on these savings to customers. (It's still not super cheap for us though. High quality machinery doesn't come without a price)
A long time compared to other methods of forming plastics, but it's significant advantages outweigh this downside for our purposes. Multicolor and multimaterial printing takes longer because you have to swap colors / materials multiple times every layer.