These were on systems with IDE ports. Usually they had 2 IDE buses(primary and secondary). Each of the ports(buses) could have a master and slave IDE device. These wre usually HDD's and CD/DVD drives.
Also note, IDE devices have a jumper on the device that can be set to Master, Slave or Cable Select.
You want to plug the Master device into the end of the cable... The Slave device into the middle of the cable.
The IDE boot drive likes to be Master on the Primary channel.
Some optical drives do not like to be slave devices, so put it as master on the secondary cable if possible.