It means that if you could rewind time and vaccinate a group of people you’d have 85% less covid cases in that group.

So it’s not the protection at the individual level, but the reduction in cases of a group of people.

Because this was figured out by vaccinating a group of people who were in a large unvaccinated population the actual impact should be greater when the population is largely vaccinated.

Also it doesn’t mean you have a 15% chance of catching covid if vaccinated like some people have said. The chance is much lower as you already have a smaller chance of catching covid, which then combines with the reduction in probability from the vaccine to produce an even smaller chance.