Content stored as a file, rather than as a structured content object. This is usually meant to refer to images, PDFs, and other files which are uploaded to the CMS by editors.
These files are often referred to as binary files, though this technically means nothing (all files can be considered “binary” to some extent). Generally, content files are intended to mean non-text files, under the assumption that text would be stored as content objects.
In some systems, these files are called assets.