Patrick Bardill, Ph.D.:
R stands for current reproduction number, and is a measure of how contagious a disease is.
R is the average number of people a person with the disease will infect before they recover or die.
For example, an R value of 5 means that, on average, each person with a disease infects five other people. If the R value is below 1, the number of cases of the disease should drop. If the R value is above 1 the number of cases of a disease will increase.
Note that both biological properties of an infectious agent as well as the behavior of people in an affected area influence the R value. Thus, it is possible for the exact same virus to have different R values in different areas depending on how people interact.
R can change over the course of an outbreak as people gain immunity and interventions change, as well. This is important with COVID-19, as practices such as scrupulous hygiene and social distancing can and have lowered the R value.
Like any disease, the R value of SARS-CoV-2 has varied. Before public health interventions in a variety of places, the R value was as high as 3 or 4. With interventions such as self-isolation of the sick and social distancing, the R value has been brought below 1 in some places.