A mainboard, often referred to as a motherboard, is the main circuit board of any PC, MAC PC, smartphone..etc!
The motherboard is what is known as a printed circuit board and is the basis of the computer. This is responsible for the power supply of the individual components ( CPU, GPU, ... ) and enables communication between the CPU, RAM and all other hardware or components.
Each motherboard is designed to work with specific hardware, so they may not work with every processor and memory type. However, hard drives whether SSD / HD are mostly universal and work with almost every mainboard, regardless of type or brand!