Yes, that's the main part. Websites you host are run from the servers. Severs should be well maintained and secure. It's uptime should be minimum. If the server is not reliable then your web hosting service will get down. As a result, you will lose your business. If server will crash then your web hosting service will be dead with it's contents, server backup is also must for the reliable server.
So basically server reliability includes, good uptime of server, server backup, proper amount of resources to handle every single request from your websites, security.