Masks are better at preventing the wearer from spreading things than from catching. Your air droplets are gonna get stuck to your mask, so they're not floating freely to infect someone. But if someone's free floating air droplets get on your mask, there's a chance it can soak through and infect you. Which is why the best bet to prevent spread is for everyone to wear a mask. Though even this isn't 100%. Just heavily reduces the chances of spread.