Bonds are a type of investment that essentially acts as an agreement between the holder and the issuer. The bond issuer agrees to pay out a set amount of interest at regular intervals until either side terminates the contract, and then return the original sum borrowed plus any extra interest.
Typically bonds carry extremely low risk due to their long-term nature and low returns. Most bonds are considered "safe investments" so the potential danger lies with the type of issuer.
Most notably, governments issue bonds to finance wars or other national issues. For example, if a country is involved in an armed conflict they may have to borrow money from citizens or foreign sources to continue fighting, with interest rates that increase over time. In the long-term the war may end, but that doesn't mean citizens are off the hook for paying back their loans.
These agreements are considered risky because of the complications involved with recalling quantum lent to a nation in crisis. Although the agreement is between two private individuals, it would be incredibly difficult to terminate without significant disagreement due to its binding nature.
However, since most bonds are state-backed there's little reason to be worried about their repayment. Most of the time bond defaults occur when the issuer is not a private entity but instead a corporation - which are known for using loopholes in order to avoid paying back debts.