Ro is a measure of how readily a pathogen spreads. The higher, the faster and extensive the spread.
Pretty good article by the former Director of the CDC, here, with a discussion including Ro:
This “spread factor” tends to lower for dangerous pathogens as time goes by, because humans alter their behavior (less travel, quarantines, develop vaccines, better hygiene, etc.) Eventually, the virus additionally can run low on new victims to infect, and THAT slows it down.
Suffice it to say that if a pathogen can maintain an Ro of 2 or better in the face of drastic containment efforts, AND has a significant mortality rate, we have a problem.
thanks for the information