The term "header" as it's currently used in Flow is very confusing. The dictionary definition of "header" is "a word or series of words often in larger letters placed at the beginning of a passage or at the top of a page in order to introduce or categorize". This is not quite what "header" is in "Flow" - it is rather an optional description of a page, that appears under its title.
Because of this difference it is hard to translate, and hard to understand in English in the first place.
It should probably renamed to something like "description" or "sub-heading".