Kartoffel answers perfectly.

Haha - did the "hit" part escape you? Go to c_trace in the manual. There you'll find the link to the "hit" struc that holds even more cool data after a trace.
The concept of nx is the a Normal. A Normal unit vector is common in all 3D. It would truly be amazing if you have been working with 3D for so long and have no idea what a Normal are.

Here is your reading https://en.m.wikipedia.org/wiki/Normal_(geometry)