According to the CDC, Coronavirus spreads through respiratory droplets produced when someone who is sick coughs or sneezes. These droplets can land in the mouth or nose of people nearby.
Using a face mask can help prevent these droplets from getting into your mouth or nose and can reduce your risk of COVID-19.
From the CDC website, according to a study on Community Transmission of SARS-CoV-2 at Two Family Gathers in Chicago, IL:
Most early reports of person-to-person SARS-CoV-2 transmission have been among household contacts, where the secondary attack rate has been estimated to exceed 10%, in health care facilities, and in congregate settings. However, widespread community transmission, as is currently being observed in the United States, requires more expansive transmission events between nonhousehold contacts. In February and March 2020, the Chicago Department of Public Health (CDPH) investigated a large, multifamily cluster of COVID-19.
Source: Ghinai I, Woods S, Ritger KA, et al. Community Transmission of SARS-CoV-2 at Two Family Gatherings — Chicago, Illinois, February–March 2020. MMWR Morb Mortal Wkly Rep 2020;69:446–450. DOI: http://dx.doi.org/10.15585/mmwr.mm6915e1external icon.