A bond is like a tradable loan. Imagine I lend you $100. In return, you agree to pay me $5 each year in “interest” until five years have passed, at which point you pay me back the $100 that I lent you. Except, during those five years, I sell your loan to someone else – so you pay them $5 each year as well as the $100 after five years. That person can also sell your loan to yet another person (who you would then pay instead), and so on.