When Pfizer says it is 90% effective what does that exactly mean?

In other words that sounds really good but leaves 100s of questions but no real answers because how it is 90% effective has not been properly defined. If you have never learned Critical thinking or learned to ask more questions about everything you might not have this reaction. But, saying the vaccine is 90% effective really means nothing at all but hot air unless you define the parameters of what you are actually talking about.