~/.ssh/authorized_keys lists the public keys (DSA, ECDSA, Ed25519, RSA) that can be used for logging in as this user.
The file is located on the server the user wants to connect to. It can be created with ssh-copy-id.
This file is highly sensitive. Therefore, the file should not be readable by other users
The format of this file corresponds to the conventional ~/.rhosts file (one key per line).
See also
The public key found in authorized_keys is typically aslo stored in ~/.ssh/id_rsa.pub (or ~/.ssh/id_<algorithm>.pub). The remote file can be created(?), updated(?) with ssh-copy-id