I cannot recommend reading A Mathematician's Apology enough. It was written by GH Hardy and I think it's one of the best non-math texts out there to understand how a mathematician's brain works.
Regular feature flag is just on and off. When you start getting into the advance you can change it for user groups, specific users, timable so it's only enabled for a timeframe, only on a specific env. The possibilities are endless.
I make the distinction so people don't just think they can put a single if statement in and be able to handle this sort of scenario.
https://en.wikipedia.org/wiki/A_Mathematician%27s_Apology
Fairly short and beautifully written.