A terminator is a resistor that is attached to a SCSI peripheral or a SCSI card. When signals go across the bus, back and forth between devices, they need to be suppressed so they do not go astray and interfere with the workings of any of the devices on the bus. The basic rule is that both ends of the bus must be terminated. If the SCSI card is the last thing on the bus, on either end, it too must be terminated. Termination applies to both devices and cards.
Terminators take different forms.
They may be actual physical resistors that you need to plug and unplug manually, depending on where on the bus your device is to go. Some devices have no provision for termination, in which case you must use a cable with a terminator built into it and the device must necessarily reside at the end of the SCSI bus. Another thing that is important to remember about SCSI is that total length of all cabling, both internal and external, is limited to 6 meters or 19.6 feet. Fast, fast and wide, and Ultra SCSI have even more stringent requirements. Fast SCSI and Fast and Wide SCSI limit the overall cable length to 3 meters.