Kubernetes Operators are software extensions that make use of Kubernetes APIs to extend behavior. In short, Operators are patterns that extend the behavior of the cluster without changing the Kubernetes code. Its APIs act as custom resource controllers.
The Kubernetes project defines "Operator" in a simple way: "Operators are software extensions that use custom resources to manage applications and their components".
Kubernetes Operators are controllers for packaging, managing, and deploying applications on Kubernetes. In order to do these things, the Operator uses Custom Resources (CR) that define the desired configuration and state of a specific application through Custom Resource Definitions (CRD).