An authentication key allows your server to authenticate the computer you are using has the right key and should be granted access. This lets you use the key instead of a username and password when using ssh.
If you don’t already have a key on your local machine (look for a file named id_rsa.pub in your user home directory under the .ssh folder
will get you to the right directory) then you need to generate the key pair. On your desktop machine use:
Next you copy the file to your server. scp ~/.ssh/id_dsa.pub [user]@[server]:.ssh/
scp ~/.ssh/id_dsa.pub username@servername:.ssh/
Rename the file on the server to authorized_keys2
mv id_rsa.pub authorized_keys2
The key is for to authenticate your computer. But on the ssh login Ubuntu will look in the user folder. So if you also had user2 access to the server and tried to ssh into the server you would not be authenticated because it would look in user2/.ssh for the authorized key file and not find it. You can put the same key in any user folder on your server to have that user also be automatically authenticated.