Click on the image to expand into a new page. These are formal and informal F stops. Each number that’s on bold is a true f stop. The others, the in between numbers, are smaller or larger increments. Why is this important? Well because F stops work in pretty well with shutter speed. It means you can work with certain shutter speeds based on the mathematics.
This chart above means that if you have correct exposure at F5.6 and 1/250th of a second, then by moving left or right by one f stop and shutter speed, you should have the correct exposure again. Look for the pattern to help you remember this.
I know it’s confusing and you can feel like a cup of a tea and a lie down when you are done, but believe me, it does get easier.