This handle is needed whenever you want to access a resource of this type later and is most of time stored in a global static variable within your extension.
To create a new resource you need to register a resource destruction handler for it. Since you can store any kind of data as a resource, Zend needs to know how to free this resource if its not longer needed. This works by registering your own resource destruction handler to Zend which in turn gets called by Zend whenever your resource can be freed (whether manually or automatically). Registering your resource handler within Zend returns you the resource type handle for that resource. This handle is needed whenever you want to access a resource of this type later and is most of time stored in a global static variable within your extension. There is no need to worry about thread safety here because you only register your resource handler once during module initialization.