Not an expert, but I believe it means that the country owes you money, to be paid back on some schedule, and in particular you can re-sell the object-that-says-

Not an expert, but I believe it means that the country owes you money, to be paid back on some schedule, and in particular you can re-sell the object-that-says-you're-owed-money to other people and therefore there is a market value for it. It's like holding a mortgage - if I get a mortgage on a house, I owe the value of the mortgage to my bank, but only slowly. My bank can then resell the mortgage to another bank, who becomes the holder of the mortgage. They're owed money by me, which is good if they think I'm creditworthy and the interest rate on the mortgage is profitable for them, and bad if not. You might want to sell a debt to someone else if you want some fixed amount of money now instead of a greater, probably riskier amount of money in the future.