A well-fitted mask, used properly, does three things:
- Filters out small, aerosolized water droplets with a virus in them (like from a sneeze or cough).
- Stops your own droplets from spreading to others.
- Keeps you from touching your nose and mouth, two vulnerable points that can both spread the virus to other people and expose you to the virus.
None of these can 100% stop Covid-19, but all of them help to lower your risk according to studies like this one performed by Texas A&M, especially alongside other precautionary measures.