The combination of RAM and swap files is known as virtual memory. The use of virtual memory allows your computer to run more programs than it could run on RAM alone. Depending on your installation method, XigmaNAS might ask you to create a swap-partition. If you don't create a swap-partition (on embedded USB/CF installation for example) you can now choose a complete drive or a single file on one of your storage devices. A good size is 150% of your RAM. In most cases a 1GB swap-file should be enough.