What is vaccine efficacy?

Since the pandemic started, we have all had to learn a lot about virology, medicine, immunology, etc. to keep up with the news about COVID19 to make informed personal decisions about their health and behavior to live safely through a pandemic.

Now, we have vaccines and "efficacy" is a big word being thrown around. 90% efficacious vs. 72% efficacious - what does this mean?

I think this article does a great job explaining how the numbers behind all of the vaccine news are generated.

https://www.nytimes.com/interactive/2021/03/03/science/vaccine-efficacy-coronavirus.html?smtyp=cur&smid=tw-nytimes&fbclid=IwAR3cd6sTEnGmWqLgwwgmNu84MJ_rQVOZPmLPd8Y0HiR6sVGYmLmdUx-ZRAI