A swipe file is your notes on a give topic. Say you do a lot of research in an area, and you keep all of your notes in a file. This would be your swipe file. Next time you want to write an article, review your swipe file for ideas. Also, it's a good place to save all references and any statistics that will probably be forgotten. It's a great thing to have lying around.
Often times, I find I create one as I research one topic, but I find some other info on the subject that doesn't quite fit into the articles I'm currently writing, but it might fit into a later one.