HD basically means it supports a resoloution capable for HD viewing.
To try it, go into Display Properties. Right click desktop and go to Properties, and select Settings from the tab.
Set the resoloution to these:
720p or 1080i
720p is in other words 1280x720 I believe, and thats the mid form that most people will use in HD.
1080i is in other words 1920x1080
If your monitor can support any of those, your monitor is in some way HD compatable. If it supports both of these, its whats known as HD Ready!
Monitors have been able to go to HD resoloutions for ages though. Its only TV's now that are just starting to get them. People have used HD in games for a while on PC for example if they've had a good enough video card.
I hope this helps