Master/slave is a model of communication for hardware devices where one device has a unidirectional control over one or more devices. This is often used in the electronic hardware space where one device acts as the controller, whereas the other devices are the ones being controlled. In short, one is the master and the others are slaves to be controlled by the master. The most common example of this is the master/slave configuration of IDE disk drives attached on the same cable, where the master is the primary drive and the slave is the secondary drive.
The master/slave model is commonly used in the technology industry, not just in electronic but in mechanical as well. In electronic technology, it is often used to simplify communication like, instead of having a separate interface to communicate with each disk drive, we can connect most of them via one interface and cable and the computer only has to communicate with one drive serving as the master, then any control command is simply propagated down to the slaves from the master.
In mechanical technology, the term can refer to the configuration of motors such as two motors connected to different drives that are acting on the same load; one drive is defined as the master, doing the speed and control of the load, whereas the slave is there to help increase the torque. In pneumatic and hydraulic systems, we also have master cylinders, which feed the pressure and control the slave cylinders.