Mailing the story:

Swap is a space on a disk drive that is used when the amount of physical RAM memory is full. When a Linux system runs out of RAM, it moves the inactive pages from the RAM to the swap space. Swap space can take the form of either a dedicated swap partition, a swap file, or a combinaton of partitions and files. Generally, when running Ubuntu on a virtual machine, a swap partition is not present, and the only option is to create a swap file.