Multiple accounts are used to check for messages from more than one email address and/or mail server. Select Settings → Configure KMail... and click on the Accounts page to add or change your account settings. See the Getting started section for more information on the settings in the Accounts page.

To check for messages from a particular account, use the File → Check Mail In submenu to select the account to check for mail. You can also press and hold the left mouse button on the Check Mail icon (on the toolbar) for a few seconds to get a list of accounts.