Etcd is a distributed, consistent key-value store used by Kubernetes to store the configuration data of the cluster, acting as the primary datastore for the control plane components. 

Etcd is written in Go programming language and is a distributed key-value store used for coordinating distributed work. 

So, Etcd stores the configuration data of the Kubernetes cluster, representing the state of the cluster at any given point in time.

https://www.redhat.com/en/topics/containers/what-is-etcd#:~:text=etcd%20(pronounced%20et%2Dsee%2D,systems%20or%20clusters%20of%20machines.