Server Sync allows you to clone all your sites from your live server to a stand-by server. The clone operates every hour on the hour.
If the primary server becomes unavailable, you can point your DNS to the backup server.
If your DNS provider is a service like CloudFlare, the DNS propogation time becomes very low and your backup server goes live almost immediately. The only data that will be missing is the new data between the time of the last clone and the primary server failure.
This is not a true high-availability solution but it does protect you from some other things such as human error.
This add-on function will NOT work with Amazon AWS EC2, AWS LightSail or any other provider where the default user is not “root”. At least not out of the box. In order to make it work you will need to forcibly enable the “root” user. Some server providers disable this user and provide a user called “ubuntu” (or with another name) with sudo privileges instead. Unfortunately, this add-on will not work with that user.