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Bonds are temporary grants of money to your airline that lasts for 5 years until you have to pay them back. They are similar to loans, but different in ways.

Each airline has it's own credit rating, depending on how often it leases aircraft, pays it's monthly lease, current bonds, etc.

Bonds have an interest rate paid every month, if you don't pay the interest (negative cash balance) you receive a warning and must pay for it 12 days later and pay slightly more than usual (negative cash balance will cause another warning).

Attention : Bonds may seem low-risk, but after 6 warnings your airline will be forced to close, and bonds have been known to even shutdown large airlines after consecutive warnings after expansion.