What does markup mean?

Answer 1

Markup describes a certain structure of a document. There is no code executed. To execute code you have to add Javascript to your HTML.

Answer 2

I wonder the same thing. But thinking about it I guess it literally "marks"-up parts of a page or app so that those parts can be manipulated by code such as javascript, php, java, etc. or beautified by css.

For example it can mark where the text or other content "is"; where or how can it be referred to by an scripting or styling language.