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.
Just go to https://webmail.123host.com.au or https://webmail.(YourDomain) - it is the same outcome - where you can log in using your email address and password.
You'll see RoundCube on the left, that is the webmail software.
If you use webmail regularly, you might want to check the box "Open my inbox when I log in" to jump straight to webmail.
You can also adjust many email settings in there.
Once in RoundCube 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