That is converting the ASCII character that was entered to a binary number that can be multiplied by another number. Strange way to comment what is being done.
Paul
look at an ASCII table and you'll see that the ASCII value for a '0' is 0x30, '1' is 0x31, '2' is 0x32. so subtracting 0x30 or '0' from an ASCII char in the range '0'-'9' will result in the value of the digit (e.g. '2' is 2)