While I was installing Ubuntu 13.10, it asked me to make a swap partition. I made it ~5.6GB, as I didn't know what it was or anything.
Simply said it is an additional space provided for the operating system to increase it's operating memory. It allows for running more software at once or to operate with larger data.
Something alike also exists in windows, only they implement it as file on a new file system, which is likely done for efficiency.