I think, by and large, rooting is an easy procedure, because some people have developed very easy-to-use tools to accomplish it. You'll have to look up the precise software for your HTC, but I think it'll be pretty easy for you to find.
Rooting is the process by which you achieve "root" (administrative) privileges on your phone, which could allow you to remove the default factory software, install a different OS, use applications that require more hardware controls, etc., etc.
Decide what you want to do with a rooted phone before you do it, though. Make sure you back everything up before you do it, and I think rooted phones lose their warranty, but you'll have to check up on that.