What are the rear filters for on a mirror lens, and how do I use them?

The second two filters you have listed are not UV filters but neutral density filters. They are used to reduce the amount of light reaching the sensor by a set number of stops, to allow for use of slower shutter speeds than would otherwise be possible. The names ND2X and ND4X stand for 'Neutral Density 2x' and 'Neutral Density 4x' - indicating that they will halve and quarter the light respectively (i.e. darken by 1 and 2 stops respectively).

The Skylight filter will reduce the amount of UV light hitting the sensor, however it is of more use with film photography where the blue-sensitive part of the film was also sensitive to some wavelengths of UV light. Modern digital cameras are calibrated not to respond to UV so it won't have an effect on your images. A skylight filter is also slightly pink tinted to remove the colour cast caused by shooting under a blue sky; this is because film has a fixed white balance which can result in a slight blue cast under these conditions. Software white balance correction means this is not an issue for digital cameras.

The reason the filters fix to the rear of the lens is because your mirror lens probably (you don't state which model) has a very large diameter front, which would make filters that attach to it bulky and expensive. Filters that attach to the rear could also feasibly be used on more than one lens, whereas if you had different lenses with different front element diameters, you'd need one filter of each type per lens.