What does this syntax of switch case mean?

It's a GNU C extension called case range.

http://gcc.gnu.org/onlinedocs/gcc/Case-Ranges.html

As noted in the document, you have to put spaces between the low and high value of the range.

case 1 ... 9:
    statement;

is equivalent to:

case 1:
case 2:
case 3:
case 4:
case 5:
case 6:
case 7:
case 8:
case 9:
    statement;