What kubernetes is 

 
It is a cluster of clusters

Kubernetes is a cluster that is composed by 

Master (Control Plane)

The master node is a collection of services dedicated to control the whole cluster.

It is a good practice not to run applications workload on the master allowing the master to focus on the operation of the cluster.

Components of a Master node

Samples

Find and replace string in file

sed -i 's/original/new/g' file.txt