A bond is a debt instrument. An IOU.
Companies issue bonds to fund long-term plans that bond-buyers can get behind. The bonds usually have a pre-determined life span, are paid back at maturity out of the profits of the project, and interest is paid for out of the company's cash flow. Company bonds are an IOU based on the company's ability or otherwise to make profits.
That's a bond. They've been around for centuries.
Seeing a chance for free money, governments quickly jumped in. Governments issue bonds when they spend more than they tax. Interest on government bonds is taken out of the taxpayer's hide; and these days, the principle is paid back by governments issuing new bonds. (Rinse and repeat, and whatever you do don't mention pyramids.) Government bonds are an IOU based on future taxpayers' ability to pay.
So what the hell is a "social bond"?
It's very simple: A "social bond" is an IOU issued in return for bond-buyers giving government even more of their money. The only difference is that not every bond-buyer gets their money back.
That appears to be the point of a "social bond."
So it's like a raffle ticket, but with a "government commissioner" goosing the draw,