GitOps is a methodology that uses Git as the source of truth for your desired state of your applications and infrastructure. A GitOps agent (such as Argo CD) attempts to keep the actual state in-sync with the desired state by continuously applying the changes from Git to your cluster.
However, Argo CD has no way to keep the cluster in a stable state in case of application bugs or failures. For example, if you deploy a new version of your application that has an error, Argo CD will still apply it to your cluster and potentially cause downtime or disruption for your users.
Argo Rollout is a Kubernetes operator that extends the native Kubernetes rollout capabilities with advanced strategies such as blue-green deployment or canary deployment.