The concept of Ro (Reverse osmosis)

Explain the concept of Ro as if you are explaining it to a colleague. What does it mean? How does it help you understand coronavirus? Why is it, from an epidemiologic standpoint, that testing for Covid-19 is important to understanding the epidemic? When you consider your particular discipline of nursing, how does an understanding of the epidemiology of Covid-19 help you in the performance of your duties?