The motherboard has a socket for the CPU, RAM modules and add on cards. In the above picture the CPU socket is the large one in the center right. The RAM sockets are the two longish light blue ones in the bottom of the picture.
The motherboard also has sockets into which you plug in your permanent storage devices like hard disk drive and solid state drive. Peripheral devices also plug into the motherboard. You can see the sockets for the peripheral devices at the top right of the picture.
The most important part of the motherboard is its chipset. The chipset determines which CPUs and addon devices the motherboard supports.
The motherboard is the foundation of your computer system. That is also why it’s called the motherboard. You have lots of circuit boards but only one motherboard.