each polygon has a normal, a normal is which was it faces. There is one side of the polygon that is suppose to face out. The side that has the normal on it is the side that should be pointing out away from inside the model.

sometimes when you are working with polys and you have 2 different objects one of them could be inside out. that will stop you from merging vertex's to sew them together after you combine them. it will either stop you or it will flip one of the polys over. So if you have trouble. Reverse the normals on one of the object. If you don’t have trouble, you don’t have to worry about the normals.