What is Termination?
Termination, in its most basic sense, is the connection of a cable to a source or destination device. For low frequency applications, this may involve simply putting the individual wires in a connector and attaching the connector to the device.
At higher frequencies, termination can also take a second meaning. To first understand termination you need to understand impedance. (A good beginning paper is Characteristic Impedance of Cables at High an Low Frequencies).
In many systems, such as Broadband/CATV or broadcast video, signals are split to go in different directions. Each of the splits (outputs) must "see" the correct impedance. Normally, one would simply attach a cable of the correct impedance to each of the outputs of the device.
But it can sometimes happen that there are more splits than desired. In that case, those unused outputs must be "terminated" with the correct impedance. To accomplish this, you attach a connector which contains a resistor inside chosen to mimic the correct impedance. In other words, you fool that output into thinking a cable is attached, when in fact, there is none!
If you do not terminate unused outputs, they can radiate, and that signal can cause noise and interference in other nearby equipment.