C# statements executes in either checked or unchecked context. In a checked context, arithmetic overflow raises an exception. In an unchecked context, arithmetic overflow is ignored and the result is truncated.

short m = 32767;   
short n = 32767;
int result1 =  checked((short)(m + n));   //will throw an OverflowException
int result2 =  unchecked((short)(m + n)); // will return -2

If neither of these are specified then the default context will rely on other factors, such as compiler options.

The keywords can also create scopes in order to (un)check multiple operations.

short m = 32767;
short n = 32767;
checked
{
    int result1 = (short)(m + n); //will throw an OverflowException
}
unchecked
{
    int result2 = (short)(m + n); // will return -2
}