cast of Objective-C pointer type 'NSString *' to C pointer type 'CFStringRef' (aka 'const struct __CFString *') requires a bridged cast

When converting an Objective-C program to a Objective-C ARC, I get the error:

"cast of Objective-C pointer type 'NSString *' to C pointer type 'CFStringRef' (aka 'const struct __CFString *') requires a bridged cast "

The code is as follows:

- (NSString *)_encodeString:(NSString *)string
{
    NSString *result = (NSString *)CFURLCreateStringByAddingPercentEscapes(NULL, 
                                   (CFStringRef)string, // this is line in error
                                   NULL, 
                                   (CFStringRef)@";/?:@&=$+{}<>,",
                                   kCFStringEncodingUTF8);
    return [result autorelease];
}

What is a bridged cast?