Where is my ETCD?

By definition ETCD = key value store.

Can anyone please explain what is that “key value store” ?
I can imagine it is some volume storage that is somehow attached to Master nodes and it contains some kind of data (key data?).
How can I display it and find out what is inside ?

ETCD is a database and the API server running as part of the control plane connects to it to save and read the cluster state.