A bond is basically a loan, an entity such as a company, a municipality, or a country needs money and issues a bond. The entity is obviously called the bond issuer and the amount of money it receives is called the principal. It receives that money from another entity intuitively called the bondholder, maybe a person, perhaps a company. Of course the bondholder does not just want the principal back from the bond issuer, he also expects interest on top of that, also called the coupon. Finally, it is clearly stated when the deal between the two parties ends or in other words, what the so called maturity date is. However, unlike with loans that you make to friends for example, bonds are financial instruments that can be bought and sold on the open market.
Let's assume that Max made a $15,000 loan to his friend Jack. If Max ends up needing cash quickly for an emergency, he can, of course, ask Jack to pay him back sooner. But if Jack cannot do that or refuses, Max won't have many options left especially in the short-term. If Max would have for example bought German bonds with those $15,000 instead, he could have simply sold them to another investor to get short-term capital that he needed. As can be seen, bonds are highly liquid, but this does not mean they are safe If the bond issuer goes bankrupt or something very bad happens, you can lose everything.