Secure Shell or SSH supports a few authentication mechanisms. If a server only supports public key authentication, a brute force attempt is near futile. This example will only look at password authentication with SSH.
To protect against attacks like these, implement rate-limiting or a tool such as fail2ban that locks out accounts for a short duration when a number of failed login attempts are detected. Also disable the root remote login. Some people like to put SSH on a non-standard port, but end up putting it on high number non-restricted ports such as 2222, which is not a good idea. If you use a high number non privileged port such as 2222, another low privilege user could hijack the port and start running their own service in its place if it ever went down. Put your SSH daemon on a port lower than 1024 if you want to change it from the default.
This attack...