Server authenticity message:

The authenticity of host 'ssh.cenits.es (193.144.255.13)' can't be established.
RSA key fingerprint is fa:83:85:6c:88:2a:6b:31:74:f7:8f:39:98:a3:75:f0.
Are you sure you want to continue connecting (yes/no)?

This message is displayed the first time you try to connect to a ssh server. Also it will be shown later if you delete the known_hosts file from your computer.

This message indicates that the public key of the server you are trying to access is not known, and you are asked to trust the server. It should be accepted to login.