(Rt) represents the effective reproduction rate of the virus calculated for each locale. It lets us estimate how many secondary infections are likely to occur from a single infection in a specific area. Values over 1.0 mean we should expect more cases in that area, values under 1.0 mean we should expect fewer.
Rt Live is tracking daily transmission rate values across the US States. My home state of Arizona is currently in a downward trend with an Rt right now of 0.77:
Looking at the daily case numbers, you can see how small rises in Rt result in muliplicative rises in case numbers:
We are noticeably flattening the curve in Arizona since late March, but opening up businesses and social gatherings any time soon will change these trends for the worse.