1. What is a 'tap cell'
The basic idea with a global substrate tap methodology is to omit internal substrate taps from the standard cells and instead to sprinkle dedicated tap cells throughout the P&R layout. The process design rules require that no piece of source/drain diffusion (inside a standard cell) be more than some maximum distance away from the tap diffusion inside at least one of the sprinkled tap cells.
If we assume (for the time being at least) that we don't have visibility into the cells showing us the exact locations of source/drain diffusion regions and/or tap diffusion regions, the tap to diffusion spacing constraints outlined above translate into a maximum allowed distance between any part of a standard cell and the edge of a dedicated tap cell.
2. What is a 'tap layer'
A tap layer is just an extra drawing layer defined in your technology file. It is not used in a fab process and will not be streamed out.
3. Why I would want a 'tap cell'
You would want a tap cell to satisfy the design rules of your technology.