Microfilm is a miniature photographic reproduction of a document. If you're looking for very old newspapers in the library, you'll probably find them stored on microfilm.
Microfilm is exactly what it sounds like: small film. These tiny photographs have to be looked at through a special viewer that magnifies them, but because they're so small, many print documents can fit on one reel of film. Microfilm is a great way for libraries to store many documents in a small space. Even the advent of the internet hasn't made microfilm obsolete — it's still a great way to preserve documents.