Resources is used to manage/store commonly used files meant for the App. For example, you may upload a logo/background images that will be used throughout your App.
You should limit the use of the files to within your own App as a good practice.
Add Resource allows you to upload a file to be added into the App.
Edit Resource allows you to manage the individual resource file. You may obtain the relative URL to access the file or make use of the hash variable given within the scope of the App for convenience. The hash variable would resolve into a relative URL same as the "URL" given.
For each resource (file) added, you can manage the permission settings. Leave the "Permission" field empty if you want to give "everyone" access to the resource. For example, "IsAnonymous" will not allow any logged in users to access the resource.