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.

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.