SBD is debt. Think of Steemit as a country with its own economy and people. Many countries have a system of selling debt in which the country is obligated to pay as long as the country is still in existence. It is a great source of revenue for the country. In the case of Steemit, the users are the primary benefactors of this system instead of a government.
In our case with STEEM/SBD, it functions in a similar way. Instead of paying users in a normal currency (STEEM), the Steemit system pays in partial with debt called SBD as long as there is enough STEEM available to cover that debt. If you are familiar with a savings bond, you can think of SBD as a savings bond for STEEM.
Excess value of STEEM covers the SBD debt. Right now, STEEM value is very low and an automatic system kicked in to reduce the amount of new debt created. The system pays out in STEEM/SP instead of SBD.
