This tab represents the current user's ignore list, i.e. a list of users from which no messages will be received. If a user who is in your ignore list sends you a message, the message will simply not be delivered.
The Add users button opens a pop-up dialog where you can select users to be added to the list. The Remove selected button removes users selected by the check-boxes from the list.
Detailed information about the Messaging module can be found in Developer's Guide -> Modules -> Messaging.