Feature flags are switches that developers architect into their web applications to allow a feature to be turned on or off. It is simple sounding in description, but harder to implement or enable after the fact.
These switches allow the systems team to operationalize new application functionality. It allows the ability to turn hot button features on or off as needed. This can be bring a tremendous power and flexibility to the operations team for deployments where traffic patterns and site usage patterns cannot be known in advance. It can increase uptime and availability of the overall site, by minimizing the impact any new feature might have.
Feature flags can also be implemented as feature dials, allowing the feature to be exposed to a percentage of users, select users, or some other meaningful way to turn it up or down gradually.