The Applications area of the navigation pane displays the custom applications activated in your domain. The applications available to you also depend upon your user/role permissions and your company's Devo service plan.
Users with Admin-level permissions can activate and deactivate applications for the domain in Administration → Applications Gallery. Once activated, the applications can be assigned to user roles in Administration → Roles. See Users and roles to learn more.
Learn more about some of the applications you can enable in your domain: