In the field of technology, slave has two meanings. The more common one refers to a second hard drive in a series of connected drives. The other meaning refers to a secondary database server that is used either as an archive or as a live backup for the main server.
Generally speaking, any sort of slave device is a secondary one that is ordered below another (called the master), which primarily responds to requests and handles most traffic. Master-slave relationships can occur in a variety of situations in the tech world, and always feature a dominant and secondary device.