It sure is and using the RoundCube email package you should find it easy and intuitive to use.
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.