A circuit breaker is a pattern used to deal with failed calls. If an error occurs, you can usually throw an exception and let the user know that something went wrong, but you can also use an alternative path to serve an alternative response. For example, let's suppose that service A has failed. Now, instead of returning a failed response, you can invoke an alternative service, B, which acts similarly to service A, in order to provide a valid response to the client, hence improving the user experience.