Birth control is known to have different %'s of efficacy. But what does that mean?

If I use a method with 98% efficacy, does it mean that if I use that method 100 times, then 2 pregnancies are may result, or does it mean that out of 100 women who use that method over a year 2 get pregnant?

Also, is whatever the answer is is the same for the %fertility rate down on <a href="https://play.google.com/store/apps/details?id=com.glow.android">glow app</a> when not using contraception?

Can you times the first by the second (so using a 98%effective method at a time when you are 10% fertile will give you a .2% chance of pregnancy?

I understand that it will be different for each individual but I am interested in what the numbers actually mean.