The study compares how many people in the experimental group and control group develop symptomatic COVID-19. In this case 11 people in the experimental group developed COVID-19 versus 185 in the control group. Thus the vaccine is approximately 185 / (185 + 11) = 94% effective, assuming both groups behaved similarly.

If the control group intentionally avoided exposure more than the experimental group, then the vaccine is even more effective than the study result. This is a blinded study though, so participants don't know which group they're in unless they go out of their way to find out (eg. taking an antibody test).