The back panel is the portion of the motherboard that allows you to connect external devices, such as your monitor, speakers, keyboard, and mouse. As can be seen in the picture below, the back panel is on the edge of the motherboard. When installing the motherboard, this side of the motherboard would be on the back side of the case and is inserted into the cases I/O plate.
The picture below is an example of what the connections look like on the back panel. If you've never seen a motherboard, you can identify these as the connections found on the back of every computer.
Additional information about each of the connections on a back panel is available on our connections page.