Gold and silver certs are negotiable debt instruments, covered by the government's power to borrow. You lend them a gold or silver coin, and they write you a note promising to repay the bearer of the note one gold or silver coin of equal weight on demand.

It's sort of a bogus loan, with a variable term and zero interest rate, but it's still allowed, and certainly makes things more convenient for a lot of people. And the government can still profit from the seignorage from the mint.