I've never been able to comprehend the idea that money is debt. I know that it is, but I fail to comprehend why. Can you explain that more deeply?

Money is debt because debt is another word for expectations. You expect to be able to trade that money in the future for at least the equivalent purchasing power you could have had now. Since SBD maintains a close peg to the dollar, then it represents a debt of future steem that must be repaid. More steem if the price of steem in dollar terms goes down and less steem if the price of steem in dollar terms goes up.

SBD is literal debt. At any point someone can convert SBD to STEEM. When they do this, the SBD is destroyed and the market must then print new steem. If the dollar value of steem has been dropping, then more steem is printed than was destroyed and this puts pressure on the price of steem to go further downward. It feeds a vicious cycle. By promoting posts you burn SBD and take some of the pressure off the price of steem since this is a debt that no longer needs to be repaid.

It is the network that pays this debt. But each and everyone of us, is the network.