Markup refers to the sequence of characters or other symbols that you insert at certain places in a text or word processing file to indicate how the file should look when it is printed or displayed or to describe the document's logical structure. The markup indicators are often called "tags." For example, this particular paragraph is preceded by a:
(or paragraph tag)
so that it will be separated by an empty line from the preceding line.
There is now a standard markup definition for document structure (or really a description of how you can define markup) in the Standard Generalized Markup Language (SGML).
Markup can be inserted by the document creator directly by typing the symbols in, by using an editor and selecting prepackaged markup symbols (to save keystrokes), or by using a more sophisticated editor that lets you create the document as you want it to appear (this is called a WYSIWYG editor).