In general the difference between primary and secondary rental car insurance
offered by credit cards is who is responsible for paying the damage first.
With primary coverage, there's never an issue if you have any other coverage
and DC, for example, pays up immediately.
With secondary coverage, you will be billed and will have to pay for the
damage (including loss of use of the vehicle by the rental car company for
the number of days they were unable to rent the vehicle because of the
damage, no matter who is at fault) and then you will have to prove to the
secondary coverage company that you have no other insurance for the loss.
Only then will they reimburse you for for your actual loss per the limits of
their coverage.