Secondary coverage means that some other company or insurance company that does not declare its coverage as secondary will pay according to its policy rules and limits first and then the secondary company pays what remains according to its policy rules and limits.

If you don't have any other insurance then that secondary coverage will cover everything that it is documented to cover.

A rental company may not refuse to rent to you if you decline their optional coverages unless it has published rules that permit it to so refuse.

It is not prudent to accept or drive a rental car without coverages from your own insurance company and/or from your credit card or purchased from the rental agency.