what exactly is HTML?

HTML is short for:
Hyper Text Markup Language.
In a nutshell: HTML is a written language used to mark-up a page. Think of marking-up like placing markers on a field. These markers give instruction to people who understand what the markers mean. In the case of web design, replace ‘makers’ with tags, a web page for a field and people with web browsers – like Internet Explorer or Firefox. To summarize:
markers = html tags/text
field = web page
people who read markers = web browsers