Multiple accounts are used to check mail from more than one email address and/or mail server. Select Settings->Configuration... and click on the Network tab to add or change your account settings. See the Getting started section for more information on the settings in the Network tab.
To check mail from a particular account, use the File->Check Mail in... submenu to select the account to use to check your mail. You can also press the mouse button on the Check Mail icon for some time to get a list of accounts.