A computer backplane is a printed circuit board that provides a parallel arrangement of expansion sockets or backplane connectors that are all wired pin-to-pin to form what is called a bus.  There can be many sockets connected although there is a limit depending on what type of socket is used.

Using ribbon cabling can produce the same effect, but for systems that are active or dynamic this can cause long-term issues with wire damage and structural failure.  Backplanes allow for more adaptability without the risk of wear or breaking.

Computer motherboards can be classified as a backplane. A specific backplane is called an active backplane, which means that the same circuit board also has logical circuitry on top of the direct electrical routing components.  The bus system on a motherboard which connects RAM memory and expansion slots use the same methodology.  Backplanes can also exist in a passive mode, which means there is no, or very minimal, logical circuitry built directly onto the board.  These are simpler and allow for more sockets and also are much cheaper to replace in case of failure, which is why they are often used instead of the active version with similar backplane connectors.