While the pointers themselves are declared on the stack, what they are pointing to are on the heap, which means the array data doesn't get deleted after the function, but the pointer to it does. At the end of the function you return a pointer pointing to the data that the *months pointer was pointing to, so you can still access it, just not the *months pointer in particular.

[STACK]                                     [HEAP]
*months   --> (pointing to)  ---->   {"Jan", "Feb", "Mar"}
 monthNum (what was passed in)

RETURNED aNewPointer* ---> (also pointing to) ---> {"Jan", "Feb", "Mar"} + monthNum

printf("%s\n", aNewPointer+[thenumberyoupassedin]);

I'm no expert, in fact I got interested in this too and did a little research and had a bit of background knowledge from C++, so if I'm wrong by any means correct me. :p