"What does 95% efficacy actually mean?"

This a great article by LiveScience, explaining some of the nuance when comparing Johnson & Johnson to either Pfizer or Moderna: COVID-19 vaccines: What does 95% efficacy actually mean?

"All three vaccines were 100% effective at preventing severe disease six weeks after the first dose (for Moderna) or seven weeks after the first dose (for Pfizer and Johnson & Johnson, the latter of which requires only one dose). Zero vaccinated people in any of the trials were hospitalized or died of COVID-19 after the vaccines had fully taken effect."