Blue-green deployment keeps two versions of a service available: the current one and a separate prepared version. It is like setting up a second stage before moving the audience over to it.
A team can check the new website version in the separate environment, then direct visitors to it when it is ready. If a serious problem appears, traffic may be switched back to the earlier environment.
This approach needs extra resources and a careful plan for changes to saved data. Switching versions is not always a simple undo if both have changed the same information.