What does "Warranty of a service" mean?

A warranty of service means that a customer will be assured of certain levels of availability, capacity, continuity, and security. However, a warranty in general just means that something is protected to a certain point. It’s like the case on a phone.

Without the case, your phone is probably going to get cracked from routine usage and a few drops. If the case doesn’t have a screen protector, you’d better hope the phone falls flat on the case instead of the screen. As long as the case is on the phone, it’s protected against anything that could happen to it to a certain degree.
The same is true of a warranty of a service.