Complex sentence: The dot or node of the Coxeter-Dynkin diagram itself represents a reflection generator while the circle around the node means the generator point is not on the reflection, so its mirror image is a distinct point from itself.
Split into simpler sentences: The dot or node of the Coxeter-Dynkin diagram itself represents a reflection generator. The circle around the node means the generator point is not on the reflection, so its mirror image is a distinct point from itself.