This is one triangle viewed from opposite sides:
3 points in 3D space are defining a triangle. In addition, if those points are ordered (like here) they are also determining two sides (faces) of a triangle : clockwise and counter-clockwise (they are determined by the order clock put inside a face would point its vertices)
In above example triangle is textured counter-clockwise
Now imagine this triangle is an element of a dice:
If you roll the dice (or rotate camera around it) our triangle from CCW becomes CW and we don't want it to be textured any more.
This dice would be in fact built out of 12 such triangles and if we order them properly we will be texturing only 6 that are facing camera at a time.