The Wilson 859922 Combiner/Diplexer is a coupler which allows you to combine 800MHz/1900MHz into a single signal or vice versa. It is most often used to combine signal from two single frequency amplifiers to be fed into a dual band amplifier. It can also be used to split signal from a dual band amplifier into 800MHz and 1900MHz components. This could be used if you have two areas with people on different cellular networks, although this is rare.
The splitter has N Female connectors. Typically a 2ft cable is used to connect the diplexer/combiner to the amplifier. Any cable length can be used, but the total cable length in the system should be kept as short as possible to minimize attenuation. Lengths of coaxial cable can then connect the diplexer/combiner to the external or internal antennas.