Co oznacza ten fragment kodu?

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        TMBaseParser *parser=[[TMBaseParser alloc] init];
        parser.delegate=self;
        NSString *post =nil;
        NSData *postData = [post dataUsingEncoding:NSUTF8StringEncoding];
        [parser parseForServiceType:TMServiceCategories postdata:postData];
    });

proszę wyjaśnić to krótko