Saponins are a diverse group of glycosidic compounds widely distributed in the plant kingdom, which are characterized by structures containing triterpene or steroidal aglycones and one or more sugar chains. More at http://www.yourhealthremedy.com/nutrients/saponins-health-benefits/

I take that last para back... I mixed up saponins with surfactants (which are found in lung tissue, to reduce surface tension that eases 'breathing in premature babies..)

Saponins as the name may indicate are compounds that have the property of dissolving fats (think soap, we use soap to wash off greasy hands !), This very property on the cellular level is makes it possible to reduce cholesterol (which is basically FAT). The cell walls of microorganisms consist of fatty chain molecules and saponins may have the ability to rupture these walls which, as SKK indicates gives rise to anti microbial & anti fungal characteristics.
From a physiological perspective, saponins are found in the lung tissue where they decrease surface tension of fluids within (premie babies are often administered this to help lung function)

Saponins are phytosterol compounds that occur naturally in some plants. Saponins have antimicrobial and antifungal properties, along with anti-inflammatory and immune-stimulating properties. Saponins are in several foods, making it easy to add this compound to your diet naturally. The good news is that red wine has a high level of naturally occurring saponins, as well as legumes and soy beans.

http://www.livestrong.com/article/333022-foods-containing-saponins/ has a good article about this.

It is a chemical found in plants (or red yeast) that some people believe helps lower cholesterol.