The term markup language usually refers to a set of characters that a writer inserts into a file of text to indicate some sort of special meaning or action to be taken at a point in a documentation.
For example, one type of markup might have semantic meaning whereby a text has notations indicating here is the title of the document, here is begins the next chapter, here is the beginning of a new paragraph, etc.
A very common type of markup is presentation markup. This is the type of markup we use on this wiki. With presentation markup, you indicate what special actions should be taken during transformation of the text from raw text to the form presented to the user. Here on the wiki, we can
and more.