Under Admin Bar > Settings > Enable/ Disable Apps, you can make Apps available to Everyone (Public), available to Site Members (Private) or Disable them. To disable an App means to remove it from the site. Even if you disable an App, the content you entered will be saved in there in case you want to make it avilable to everyone again. When you turn on an App, this affects what content you can add to a course. If you want a media gallery, you can simply turn on that App, and then in your Front page (home page) under the Add New button, you'll see you can now add a media gallery. If you want to make a new page, either make the Pages feature available to Everyone or just Site Members and you can now add a Page to your site from the Front page. You can also add new app items by going to Admin Bar > Site Content > Add , you can click on an App and it will create a new app item on your site.
You can customize most Apps by clicking on Admin Bar > Settings > App Settings. This is useful in certain Apps like Publications because you can change the bibliographic format and how many items are displayed through the Customize link.
Whenever editing Apps, always click the Save button to save the changes.