The main points of a presentation would be the big points that you want to make.
For example, in a presentation about Africa, you may mention some general characteristics for the whole continent. This could be the climate, the traditions or the history of the continent.
For sub-points you will want to go into further detail about your big points.
For example, in the same presentation about Africa, you may mention specific countries. Africa is a huge continent filled with all different types of people. You would be required to go into detail about this information and bear in mind the nuances and differences between each and every country.
In this case, you are going from discussing issues on a broad scale to discussing issues on a narrow scale.
