Being a visual learner, I find explaining SCSI termination like a race track helps visualize the need and use of termination.
Imagine SCSI sends data only "one way" like a single lane road or on a race track. The only way to get the data to "turn around" is to add a U-Turn. That U-Turn is the terminator at one end on the cable.
Now that you have the data turned around, you need to get it to turn again to complete the circuit. So you add ANOTHER U-turn/terminator at the other end of the cable.
Doing this allows the Data to race around (read/write) at full speed without slowing down because the U-Turns are made to redirect the flow without it slowing down.
Not putting on terminators causes’ confusion that then slows down data transfers, causes errors or even fails a drive that is working perfectly. I've run systems without proper termination and believe you me; I have nothing but dumb luck to thank!
Someone mentioned terminating every unused spot on the cable. Using the same analogy, terminating/placing a U-Turn will cause your data to turn around and go no further down the cable effectively cutting off any other devices connected past the terminator.
Hope this helps clear it up a bit.
