SSH.. keybased logins?

Håller på att installera rsnapshot som är ett program för att göra backup. Rsnapshot kan göra backup till en annan remote server med hjälp av ssh. Men då måste keybased logins vara aktiverad. Vad menas med detta? Och hur gör man?

Så här står det på HOW-TO sidan:
http://www.rsnapshot.org/howto/1.2/rsnapshot-HOWTO.en.html

You must have key-based logins enabled for the root user at example.com, without passphrases. If you wanted to perform backups as another user, you could specify the other user instead of root for the source (i.e. user@domain.com). Please note that allowing remote logins with no passphrase is a security risk that may or may not be acceptable in your situation. Make sure you guard access to the backup server very carefully! For more information on how to set this up, please consult the ssh man page, or a tutorial on using ssh public and private keys. You will find that the key based logins are better in many ways, not just for rsnapshot but for convenience and security in general. One thing you can do to mitigate the potential damage from a backup server breach is to create alternate users on the client machines with uid and gid set to 0, but with a more restrictive shell such as scponly.