Swap space is space on your hard drive that is reserved to supplement your memory. When there is not enough physical memory in to store all the running programs, then programs that aren't being actively used are moved to the swap space.
Swap is also known as page file (PF) or virtual memory (VM).
In short it is junk from the past and with the prices of ram being so low plus since it's using the hard drive you REALLY don't want to be using it since that is where your client's (or YOUR) files are being loaded from and sent to the clientside (people surfing). Hard drives about 100 times slower then memory and thus you should really notice a slow down then your server starts using the swap (assuming it's correctly configured and not intially going after the swap before the physical ram). You can buy 1GB of acceptble ram for about 95 bucks on respectable online stores, sometimes less.
If your OS (like Linux) is responsable and doesn't go directly for the swap before the physical memory (windows in example) then it's ok to have in case you get someone who is doing something they're not supposed to.