What do we mean when we say “static content”?

Static content is defined as content that is served as-is to all users when they request it, as opposed to dynamic content that must first be produced and is usually different per user.

The static files proposed to be cached are the following:
-- Images: png, jpg, gif, jpeg, etc.
-- Text files: css, javascript, html etc.
-- Documents: doc, odt, pdf, xls etc.
-- Setup files: bin, exe, deb, etc.
-- Compression files: zip, 7z, tar.gz, etc.