I think people are meaning
1- way too much bow in the neck (downward curve)
2- body join is usually high (upward curve)
3- fall away to sound hole (2nd downward curve).
I have never heard it used in a good way. Necks are supposed to be straight if it has an "S" shape they are probably saying the neck has warped and is either unplayable or borderline unplayable.
I think people are meaning
1- way too much bow in the neck (downward curve)
2- body join is usually high (upward curve)
3- fall away to sound hole (2nd downward curve).