running head
A running head is the line repeated at the top of every page to say where in the document the reader is.
In more detail
Open a long report in the middle and the first question is what am I looking at. The running head answers it without the reader going back a single page. In a book it is usually the title on one side and the chapter on the other; in a report, the name of the report and the section.
It is furniture rather than content — something the page is furnished with, not something the document says. It is not a heading, it opens nothing, and a table of contents ignores it. The page number and the rule drawn under it are furniture too.
What it does on a real document
A contract of forty pages, printed and lying unstapled on a desk. Without a line at the top saying which contract and which article, a page that has slipped out of the pile belongs to nothing and nobody knows where to put it back.
Inside the file, a running head is marked as furniture. Software reading the document aloud then steps over it, instead of announcing the title of the report at the top of every page for forty pages together.
Where it comes from, and why
Manuscripts carried them before printing did: a scribe wrote the name of the book across the top of the opening so that a reader could find their way in a volume whose pages bore no numbers. The printed book kept the habit and added the number.
Head is the printer's word for the top of the page, and the line runs across every one of them, which is where the name comes from. What is new is only that a file now has to say the line is furniture, so a machine does not read it out as text.