Mucks with the SAM's video base address, so that the data showing on the screen can come from someplace other than the chunk of memory allocated for that by Color BASIC. Lets you browse around, looking at different pages of RAM.
Starts out by resetting the video base address to zero, showing you the messy (but interesting) page of memory in which Color BASIC keeps many of its internal pointers and stuff. Lets you see different "pages" of RAM, where each page is 512 bytes long, and page N's base address is N*512. (So pages are numbered 0 through 127.)
You can see different pages of memory by pressing the up- or down-arrow keys, or by holding down the shift key while doing that. Up-arrow and down-arrow show you the next higher and next lower pages of memory, respectively. Holding shift plus one of those steps you ten pages in that direction, instead of just one.
Each time you change pages, it shows you the new page number, just for a fraction of a second, before showing you that new page.