For signed integer types, e.g. short, int, long, positive values (and 0) are represented with the leftmost bit equal to 0.

For any such representation of a positive value, the two's complement, which is the corresponding negative value, will have a 1 in the leftmost bit.

So the left most bit is the "sign" bit. A sign bit of 0 means the number is 0 or positive; a sign bit of 1 means a negative number.