From the this tab, you can choose which parts of the CMS Desk interface will be displayed to members of the given role. You can also change the settings of the WYSIWYG Editor for this particular role.
Individual interface elements can be configured from the Dialogs sub‑tab. This is where you can select a module from the Module drop-down list and then enable or disable the visibility of individual dialogs by checking the corresponding boxes. Only dialogs that are checked will be displayed to users assigned to the given role. You can easily reach nested dialogs by clicking the Expand all and Collapse all links, respectively. You can also expand and collapse dialogs on different levels of the dialogs tree. If you would like to adjust WYSIWYG Editor settings for the given role, switch to the Editor tab. You can make adjustments in a similar way as with dialogs.
More resources on Roles can be found in Developer's Guide -> Development -> Membership, permissions and security -> Security model overview.