So you still need an explanation of the SBD? So OK, here it is.

(this painting is from the scene of the "Full Metal Jacket" movie)

First of all, I was thinking why - why you cannot google (internal search for it) by yourselves.
But I found an answer: there is no spread the meaning of the abbreviation.
Interpretation of the abbreviation.

Even in the Steemit FAQ:

Steem Dollars - Steem Dollars (commonly abbreviated SBD) are liquid stable-value currency tokens designed to be pegged to $1 USD. Steem Dollars can be traded with STEEM, and transferred to other accounts for commerce or exchange. Steem Dollars may also be converted into STEEM via a process that takes 3.5 days.

SBD means Steem-Backed Dollar(s)! It is not a real token itself! It is the Steem Blockchain
guaranteed dollar equivalent (for a rather peculiar conditions):

also by Steemit FAQ:

"How do Steem Dollar to STEEM conversions work?"
If you convert Steem Dollars to STEEM, the blockchain will process the transaction over a period of 3.5 days. At the end of the 3.5 days, the SBD will be gone and replaced by approximately $1 USD worth of STEEM tokens. The "approximately 1 USD worth of STEEM tokens" is based on the median STEEM price over the 3.5 days, using the price feeds from the Steem witnesses. Depending on price fluctuations during the 3.5 days it is possible to end up with more or less than $1 USD worth of STEEM per SBD at the end of the conversion. This is an advanced user feature, and is currently only available to users using external wallets.

Also Steem blockchain can convert all SBD by demand because of...
lets see https://steemd.com
virtual_supply - 271,558,957.048 STEEM
current_supply - 265,703,768.034 STEEM
subtraction between shows the margin for SBD guarantee convertation

Also I wanna show you what @ned answer for
"How can Steemit back the Steem Dollars with US Dollars as promised?":

Steem Dollars are a blockchain based contract in which the Steem blockchain has committed to buy and destroy the contract in exchange for Steem at the value of a dollar. The system does not depend on a third party to 'back' the value. The value comes from speculation (see: Steem on Bittrex.com or its market cap on coinmarketcap.com), price feeds and recurring seignorage to reward holders of Steem Dollars.

Simple example:
Person A has borrowed money from Person B
and wrote a promissory note that Person A should return, for example, $ 1000;
Strange Person C asks Person B to sell this note for $ 2000;
after that another Person D asks C to sell this note for $ 2200
and so on...
But when anybody come to the Person A - it get only $ 1000
(read the conditions inside the note - not even court will help you!)

So
SBD is the bad way to keep your savings
'cos the Steem blockchain guarantee it's only as 1 SBD = $1.00
all above - it's your own risks and porblems,
It is own risks of the traders who don't even know where and what the Steemit social media is.

And my advice to you:
Try inner search by your own for this and any other questions.
Suddenly, it works! :D