Systems Manager Session Manager allows you to log in to your instances without the need for SSH keys or Terminal clients. This allows you to rescue instances if you happen to lose your key pairs, and it can add an increased level of security since, if you never create a key pair in the first place, there is no chance that it could ever be compromised.

The SSM Agent, which is installed by default on Amazon Linux instances, combined with the instance profile that is attached to the instance, allows Systems Manager to take actions on that instance on your behalf.