Saponins are a type of complex carbohydrate (glycoside) which contain several sugar molecules attached to a polycyclic carbon backbone (Aglycone). Saponins serve several important functions in the plant such as IPM, wetting abilities (for soil) and even soil rejuvenation. They are found in most vegetables and herbs, having a bitter taste and being toxic to insects. It is the plants natural way of fending off bugs.
Saponins are amphipathic, meaning they work in both polar and non-polar conditions (water and oil). Due to this along with their highly conjugated sugar side-chains, saponins are able to cause foaming / frothing characteristics in water. This causes water to penetrate the roots more efficiently, and will allow you to get more optimal nutrient uptake, and go longer in between watering.
Furthermore Saponins are able to break down into their Aglycone Carbon Backbone, which is toxic to insects and acts as an effective insecticide (especially in combination with Aloe and Agsil etc).