Webmail is for people who want to check their mail on the web or "in the cloud" as the new trendy term goes. Think of it as like using Gmail or Hotmail.
Head to webmail.YourDomainName.com.au enter your full email address and of course your password, click Log in
You'll notice there are a few options for you to explore, we're going to stick with reading email here.
This image is not up to date, you will only see Roundcube because it is the easiest to use and to read. If you enable autoload it will automatically go into roundcube whenever you enter webmail, but you can stop it if you want to use one of the other options.
Head into the settings and tweak time zone, name all sorts of things. If you can't figure out how to do something try googling for "roundcube (whatever)".
You can test your email by sending yourself a message. Go on try it. Then reply.
Make sure it all looks the way you want. If not, head back into those settings.
Stuck? Lost? Inbox fever? Lodge a support ticket