A mother-board provides suitable interface and circuits for communication and power-supply among/ for the processor, working memory, I/O, persistent storage etc..
The motherboard contains the BIOS memory where the boot-program is located.
System Bus
The system bus is a common communication channel on the motherboard. It has many different dedicated branches for communication between the different components and certain controller hubs.
A computer bus operating with \textit{double data rate} transfers data on both the rising and falling edges of the clock signal.
Quality
Important factors determining quality are: the bus clock frequency, the choice of I/O protocols supported.