vaccine trials - what does the 90% or 95% effective mean?

Steven Avery

There are 94 confirmed Covid cases among almost 50,000 participants of which close to 40,000 received 2 doses. They will continue to do the study until they have 164 confirmed Covid cases.

I'm not an immunologist but can someone explain to me if I got this right: Out of 50,000 participants, in the end only 94 got Covid and between those 94, you got to check who got a placebo and who got the actual vaccine? So if 10% of those 94 got Covid even with the vaccine, does that indicate the 90% efficacy that is mentioned?

EDIT: seems like the last paragraph I've written is sorta wrong. Am still confused. I found the Pfizer/BioNTech study design protocols for I/II/III here though