What does "sum and substance" mean?

The essence or gist of something, as in The sum and substance of their platform is financial conservatism. This redundant expression—both sum and substance here mean “essence”—has probably survived owing to alliteration. Shakespeare used it in The Two Gentlemen of Verona (4:1): “My riches are these poor habiliments [clothes], Of which if you should here disfurnish me, You take the sum and substance that I have.”