Feature flags (aka Feature Toggle, aka Feature Switch) let you enable program/web site features selectively. This is especially important in continuous delivery environments where code is always committed and deployed even if a feature is not ready for prime time. It can be useful if you have a large user base and want to introduce the feature incrementally by enabling the feature for sub-sets of users. Feature flags are useful in anything beyond toy apps.
Feature flags are not new. They are widely known since Martin Fowler blogged about the pattern. He did not invent them. Many good developers have been using feature flags or alike for a long time. But Martin Fowler recognized feature flags as a remarkable pattern. Writing about feature flags was a good idea, because other developers who are not aware of the pattern can learn from it. That was 2010.
Guess what, in 2006 a lonely Weblin programmer added feature flags to the Weblin portal. Weblin has been using feature flags from the start.