Masks don’t stop the virus particles directly if they are in the air but do a good job of blocking the droplets that we expel when coughing, sneezing (or singing) that carry them. Experts (who originally urged not wearing them so there would be enough for front line healthcare workers) now agree we should wear them. See the video here to understand why. A mask in my view, protects others 80% and you 20%. Your mask protects me 80% from your virus and you 20% from mine.
PREVENT THE SPREAD: Wear a mask to prevent infection - YouTube