The 3^rd parameter, 0666, is also called the mode, but as noted above, it specifies the file access permissions.

The leading 0 means 0666 is interpreted as an octal number (base 8).

To represent one of the 8 digits in octal only 3 bits are needed. Converting octal to binary means each octal digit becomes 3 bits.

In binary 0666 would be

        6   6   6   (octal)
        110 110 110 (binary)
        rw- rw- rw- (permissions)