Show respect to people who have died by not saying anything bad about them. Since dead people can no longer hurt us, or defend themselves, it is better to forget their bad actions and remember only their good ones.
This saying may be seen in three ways: 1) We cannot be 100% sure about anything (except dying and having to pay the taxman - in this case the latter idea being added for humour). 2) We cannot escape taxation (in this case the idea of death serving only to highlight the certainty of taxes). 3) We can be absolutely sure that we will die (in this case the idea of taxes being added for humour). The first interpretation seems to have been the original intent (see Origin below).