First of all, you should place your code within code tags. You can edit your post, highlight your code, and click on the Format button that looks like "<>". For some reason this does not work when creating a thread, but you can always edit your post.
Now, your question.
The operator+() function takes a loc object, and returns a new loc object which is the sum of the 2 loc objects. For instance, if you do the following:
1 2 3
loc a(5, 10);
loc b(3, 9);
loc c = a + b;
you create 3 loc objects. The loc "c" is created by adding a and b together. Adding the loc objects means adding the piece parts of the loc objects.
The operator+() function is called on object "a" because it is on the lefthand side of the addition statement. b.longitude is added to a.longitude and stored in temp.longitude. Likewise with latitude. The value temp is returned and assigned to c.