The "..." means that the fprintf function can take any number of arguments (well, more than two, anyway). printf is the same. Consider the difference between
printf( "Hello, world!\n" );
The first example, printf takes one argument (the Hello world string) and in the second it takes seven arguments (string, plus six numbers). There is no way of telling in advance how many arguments printf takes, which is why C allows for varying-length argument lists.
printf( "Here are six numbers: %d, %d, %d, %d, %d, %d\n", 5, 7, 11, 42, 0, 32767 );