Fail2Ban is an application which bans IP addresses from logging into your Linux SmartVPS or Linux Stack Server after too many failed login attempts. Since legitimate logins usually take no more than 3 tries to happen (and with SSH keys, no more than 1), a server being spammed with unsuccessful logins indicates malicious attempts to access your Linode.
Fail2Ban can monitor a variety of protocols including SSH, HTTP, and SMTP. By default, Fail2Ban monitors SSH only, and is a helpful security deterrent for any server since the SSH daemon is usually configured to run constantly and listen for connections from any remote IP address.
For complete instructions on installing and configuring Fail2Ban, see the guide: