90% Vaccine Effectiveness Claim Interrogated

Earlier this year Pfizer made a claim that their vaccine was 90% effective. How accurate is that claim? On what did they base this claim?
This video interrogates how medicines are evaluated and how this has been applied to Vaccines for Covid 19.
What is the difference between Relative Risk Reduction (RRR) and Absolute Risk Reduction (ARR)? What is the significance of Numbers Needed to Treat (NNT).