A motherboard is a printed circuit board (PCB) that holds other modular devices, both required and additional, and facilitate communication among the devices to actually have the computer be functional.
Motherboard is like a backbone to a computer system. Components like CPU, RAM, Harddisks along with additional devices like TV Cards, graphics etc. all initially connects to the motherboard. The motherboard enables the devices by ensuring power supply to the device, transferring instruction from and to the device etc.