Extensions allow Fauxton views to be have extra functionality.
A module registers an extension by
FauxtonAPI.registerExtension('extensionName', myObjectToRegister);
Any other module wanting to use that extension can then get all objects registered for an extension by:
var extensions = FauxtonAPI.getExtensions('extensionName');
// extensions will always be an array
The module can then use those extensions to extend its functionality.