(I'm enjoying doing these raw, barely edited writeups; I hope they're useful to you too)
This is my own writeup on feature flags; for a deep dive I'd recommend something like Martin Fowler's article (https://martinfowler.com/articles/feature-toggles.html).
So. Feature flags. The basic idea that you'll store configuration/values on a database/service somewhere, and by changing those values, you can change the user experience/features for a user on the fly.
Let's say that you're building a new feature, called 'new-button' which changes the color of buttons, which is currently red, to blue. Then you'd change code that looks like this -