Backup of water is the back up of water in drains and sump pumps.
If you read that thinking, wow, that is totally not an explanation, you are absolutely correct.
What drain, well, all of them. What you mean if my toilet overflows because my kids are gross, that is covered, no. I mean drain within your house that can receive water from an outside source.
Okay, I hear it in your thoughts, I am not purposely being vague, this is how it is thought of in the insurance world, water from an outside source. (ground water, rain water, flood water)
Okay, now that you have more questions than answers, let's look at sump pump failure.. Oh you guessed it, when it does not keep up during a storm and, well, water invades your home. Not all states have basements, but if you do, this is always a recommended coverage.
Still confused, I get that, but reach out if you have specific questions.