Webmail allows you to check your email from a web browser as opposed to a desktop mail application such as Outlook Express or Thunderbird. This means that you can check your email from any web browser. Please make sure to log out when checking your email on public computers so that other people cannot read your email.
You can check the webmail for any account by logging into https://yourdomain.tld:2096 with the full username (email@example.com) and password for the email account or by clicking on the below icon.
To check your mail through a desktop application you will need to configure the application. Your mail server is most likely mail.yourdomain.tld. You will need to use the full username (firstname.lastname@example.org) and password when setting up your account in your desktop mail application. It is a good idea to check 'My Server Requires Authentication' as well as most mail servers require this option.