aam_access_dashboard
This capability is managed by the Additional Caps service that is enabled by default in the free AAM version. You can enable or disable this service on the AAM "Settings" page under the Services tab.
Warning!
Be careful. Depriving the Administrator role or your admin user of this capability will immediately lock you down from accessing the backend area of your website.
The aam_access_dashboard is a custom capability that you create manually. Upon creation, all users and roles that do not have this capability explicitly granted will not be able to access the backend site of a website (in other words, any URL that starts with /wp-admin/).
This capability is handy when you need all your users to restrict access to the WordPress admin panel except a few. You can learn more about this from the How to lock down the WordPress backend Q&A.