When creating an App with Facebook integration, it asks for an App Namespace (that only accepts 20 characters, by the way) seen here:

So, this is going to sound like a total "What are you even doing here if you don’t know this" question, but where is my "App Namespace"? Is it in my Manifest or something?
Also, can I change my namespace?
The “App Namespace” is the new version of what used to be called “App Name”. This is used for deciding what should be the URL – “apps.facebook.com/name_space” and it has nothing to do with Java namespaces.
For apps that are housed in a tab only, the namespace doesn’t matter much – since the URLs for those are formed of the template = “www.facebook.com/page/sk_app=appId” but for apps housed in canvas, the URL might be something you want to tweak to attract more visitors or to present a brand’s identity within it.
Since Open Graph has been launched, the namespace is also used for OG objects, but since OG objects are more used only by machines to read (as in, they are never exposed to users), it doesn’t really matter what namespace you select.
AFAIK, namespaces are changeable.