That is based on an event study approach from a recent Health Affairs article by Lyu and Wehby (2020). Specifically,
Mandating face mask use in public is associated with a decline in the daily COVID-19 growth rate by 0.9, 1.1, 1.4, 1.7, and 2.0 percentage points in 1–5, 6–10, 11–15, 16–20, and 21 or more days after state face mask orders were signed, respectively. Estimates suggest that as a result of the implementation of these mandates, more than 200,000 COVID-19 cases were averted by May 22, 2020.
As of the end of July, we are still at peak levels in the US for new COVID-19 cases. Thus, if you go out side where you are around others, wear a mask.