Extended Data Space in PIC24
The passage you quote of course assumes you have already read the general architecture document.  You need to go read that to understand data space addressing in general.

Briefly, the original architecture only envisioned 32 kB of RAM.  The high bit of the 16-bit address was originally used to select the PSV (program space visibility) window.  Since program memory can be much larger than this PSV window in data memory, a separate register (PSVPAG if I remember right) is used to select where in program memory the start of the PSV window is mapped to.

Extended data memory uses a similar mechanism.  There is a additional register (don't remember its name right now) that specifies what part of extended memory is mapped to the start of a special extended data space window in the data address space.