What is a channel tap?
In order to simulate the effects of multipath fading, a model can be created that consists of a delay line with several taps. A tap in this context is just a point on the delay line corresponding to a certain delay. The signals from each tap can then be "summed" and the composite signal then represents what a real radio wave might look like as received by a receiver and when subject to multipath fading.

The summing I refer to above is just the mixing of the individual signals from the various taps but, importantly, these can be summed together in different ways to simulate how time can play a part in the real mixing of the multipath signals.