The current prevailing theory is that the coronavirus is transmitted through droplets exhaled when we breathe (or sneeze, or talk, etc…). Masks create a barrier that stops the droplets from spreading as easily as when a mask is not worn.
Considering the fact that the coronavirus may take up to two weeks to cause symptoms and many people do not show any symptoms at all, it is even more important to ensure that we are not spreading the virus, especially to people who are at greater risk due to age, health and other factors.
Learn more at https://www.healthline.com/health-news/the-simple-science-behind-why-masks-work