Flat plate collectors are essentially an insulated box filled with dark absorbent material, with tubes running through it. There is also a plate of glass on the front of it that can be glazed or unglazed that the sunlight passes through, which retains some of the heat that is absorbed by the dark material.

Sunlight comes down, passes through the glass panel, and is absorbed by the dark material inside the box. That absorbent material naturally heats up as more sun hits it. There is a constant circulation of fluid through those tubes, and as the absorbent material heats up, the fluid in those tubes heats up as well. That fluid is then moved through the system through pumps (active systems) or with a thermo-siphon (passive), to be used in the household.