Communication categories are not the same as contact categories. They relate to what your contacts see when they click on the "Manage My Preferences" link at the bottom of any of your emails. *
When they click this link, the options they see on the subsequent landing page reflect the communication categories created in your account, and this allows them to opt in and out of each category.
This helps to lessen your un-subscription rate, as contacts may be happy to keep receiving emails from you, but may only want to see emails related to certain communication categories.
When enabled,* all emails created must be assigned to a communication category in the email builder (1). If an email does not match any existing communication categories, an Admin can add a new one.
* Not all accounts have communication categories enabled. In some accounts, contacts will just be directed to an "unsubscribe" landing page. In this case the Communication Category dropdown in the email builder will be hidden.