It means that on the DS the memory expansion sits flush with the case of the DS itself. On the DS lite, the memory expansion pack is slightly longer than the slot for it, so it will go in and it will work but the pack sticks out from the DS a bit. This is because the slots are different depths on the DS and DS Light. This is noted in the documentation included.
Pretty crummy browser if you ask me, I own it and you can't really type quickly on it, no flash, loads CRAZY slow (Google will load in 3-5 seconds, but a site with more than just text takes forever to load), and needs expansion memory just to work. Not that it's a bad auction, either, the web browser is just tough to use.
Good input. It certainly doesn't compare to modern devices, which is why I completely forgot about it after I got a smart phone. Hopefully the winner has more use for it than I.
My advice to the winner: make liberal use of bookmarks so you have to type less and be aware that it is probably best for casual browsing like checking (but probably not writing) email or listia actions :) I barely even use my DS anymore but for someone who does use it alot, it might be worth having in your bag.