A feature flag is a setting that lets a team switch a feature on or off while keeping its code in the app. It is like a light switch: the wiring is there, but the light can be controlled separately.
A team might use a flag to try a new page with a small group before showing it to everyone. If the page causes a problem, turning the flag off can hide it while the team works on a fix.
Flags need care: old switches can become confusing, so teams should remove ones they no longer need.