The top three entries in the navigation bar allow to manage the security features of Eclipse Mosquitto. You can define users, groups and roles. Roles define a set of rights or restrictions to publish to topics or to subscribe to topics. These roles can be assigned to groups or clients. A client can belong to a group inheriting its roles and also have roles in addition to that. A group manages the clients that belong to the group and can have roles assigned that are inherited to the clients of that group.
Follow the link to get a description of the features behind the following icons:
The dynamic security feature is available since version 2.0 of Eclipse Mosquitto. If you are still using version 1.6, see this guide on how to migrate.