A swipe file is a collection of tested and proven ads and sales letters.
Keeping a swipe file (templates) is a common practice used by advertising copywriters and creative directors as a ready reference of ideas for projects.
Swipe files are commonly used by Marketers who need to gather a lot of resources not only about products but also about marketing methods and strategies.
These collections of ads, proven words and phrases are closely guarded because they generate sales.
Successful marketers and copywriters take years to accumulate this info and usually never share these secrets with outsiders because they know how much this information is worth. Indeed, in some cases it's the swipe file that's responsible for the success of many copywriters.