Design for Manufacturing (DFM) is the process of taking an engineering design that has been validated at the prototyping level, and iterating the design to render it suitable for production using chosen manufacturing processes.
Design for Assembly (DFA) is the same process, but designing for ease of assembly to cut down on the time it takes to put together a product and improve quality and consistency.
Sometimes we group the two activities together and call it "DFM/A" - design for manufacturing and assembly, because it makes sense to do both at once when designing complex subassemblies for a product.
For example, a consumer electronics product with PCB Assemblies and plastic housings might be prototyped using fast-turn PCB services, and the clamshell housings might be 3D printed or urethane-cast. The PCB may not have a robust enough design to pass emissions testing. The plastics may not be designed to be injection moldable due to the presence of undercuts. To render such a product mass producible, a design iteration is in order to ensure all the subsystems are redesigned for manufacturability and to hit cost targets.