Dunno, if the other posts have helped, but just in case, here goes:

You N800 has 128Mb of RAM, which it uses to run programs and the temporary data they use (the web page and graphics you're looking at for example). If you open too many programs then you can use up all of that 128Mb. If that happens, as there is no more memory, you'll find you can't run any more programs, and some of the existing ones will crash (as they try and get more mem, but can't).

The solution is a swap file, where chunks of memory that aren't used any more are moved to the external device (an SD card in this case), freeing up the internal memory.

You don't want this happening too much, as swap space is inherently slower than real RAM (especially the case with PCs which use their much slower harddisks as swap space), but the extra headroom is a bonus if you're running out of space.