It is an acronym for HyperText Markup Language. It is the code that is 'behind' websites....it basically tells the internet how to make things appear and what to do.
This is a helpful website to explain things better:
http://www.arachnoid.com/lutusp/html_tutor.html
I am a non-IT person working in an IT department (on the admin side), and half the time I feel like everyone is speaking a different language. I have to look things up on the web (bloomin' acronyms) on a daily basis!