This is a quite an old (circa 1970's) type of socket / interface that is still present on the back of most personal computers.
It was mainly used to connect computers to a printer with a thick unwieldy cable. In fact it was commonly called the 'printer port'. It looks like a long blue socket with 25 pin-holes arranged in two rows. These days the USB port has virtually taken over this task.
The parallel port are still useful though for control purposes. For example, computer controlled devices can be connected via the parallel port. Control software sends electrical signals through the parallel port.
'Bi-Directional' parallel ports can also read incoming signals, for example from a data logger.
Challenge see if you can find out one extra fact on this topic that we haven't already told you
Click on this link: Parallel port