Selectors are used to filter out objects based on their assigned Labels. Could contain dashes (-), underscores (_), dots (.), and alphanumerics between.

Unless empty, must begin and end with an alphanumeric character (),.

Must be 63 characters or less (can be empty),.

Valid label keys have two segments: an optional prefix and name, separated by a slash (/). Each label key must be unique for a given object. Each Kubernetes object can have a set of key/value labels defined. We can attach Labels to objects during their creation time or can add or modify them later as well. # where environment is production labels: environment: prod app: nginx # where environment is development labels: environment: dev app: nginx # where environment is staging labels: environment: staging app: nginx In such cases we need to assign labels to those resources using their object configurations files. Let us assume we have three sets of a same application running on three different environments such as- prod, dev and staging.

When we categorize something based on their properties or characteristics and give them some name those are nothing but called as Labels. As per official documentation Labels are key/value pairs that are attached to objects, such as pods, services, deployments and replicasets etc.

Kubernetes annotations