background task identifier

[Shades of Chaos]

Hey guys,

I was trying to implement multitasking in my app and I read the documentation on in, but I don't understand something. In the docs Loading…

They use the code

- (void)applicationDidEnterBackground:(UIApplication *)application
{
    UIApplication*    app = [UIApplication sharedApplication];

    bgTask = [app beginBackgroundTaskWithExpirationHandler:^{
        [app endBackgroundTask:bgTask];
        bgTask = UIBackgroundTaskInvalid;
    }];
    // Start the long-running task and return immediately.

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

        // Do the work associated with the task.

        [app endBackgroundTask:bgTask];
        bgTask = UIBackgroundTaskInvalid;
    });
}

I do not understand what bgTask is. The docs said

The bgTask variable is a member variable of the class that stores a pointer to the current background task identifier.

But I don't really know what a background task identifier is. Could someone give me an example of one or explain it please?

Thanks