Well the way i was thinking was that the boolean keyword indicates that shortedOnPcb() is a function. Functions always have a return value, in this case either true or false, so it's a boolean function.
shortedOnPcb() will return TRUE if the two pins currently being looked at are supposed to be shorted, FALSE if they're not supposed to be. ! negates the value.
The first number in the pair will always be the output because this is the pin we set as a high, we then read a the second number to see if its high or low