A SWAP file is a section of a hard disk set aside to function as virtual RAM memory, should you run out of the actual physical random access memory - the system hits the hard disk.
Windows works in a way you really never have to deal with SWAP files or virtual RAM partitions much in regards to setup, but Amiga OS4, like Linux - you create a SWAP file upon setup of the hard disk. When you setup a new 4.1 machine, or put a new hard disk in a AOS4 machine that does not have a swap file partition, you will be prompted to set one up. IIRC, the recommendation for OS4.1 is 2x your physical RAM. My SAM 440 has 512 MB, so 1024 (1 GB) was recommended, but I think I bumped SWAP up to 1.5 GB just to be safe. And I don't think I've ever had to hit SWAP once yet, but I cranked it up pretty high once I put the SSD in. Setting up a SWAP partition in OS4 is extremely easy, no more difficult than it is in one of the popular GUI based Linux distros.
Essentially, you reserve a portion of the hard disk to act as RAM should you actually ever run the SIMM/DIMM ram chips right down and out of memory. This hopefully means you'll never get an out of memory message
