A bond is a debt instrument issued by a corporation (a corporate bond) or a country (e.g. State note in Belgium). So when you buy a bond you are actually lending money to a country or a corporation. The bondholder receives interest annually. The loan matures after a certain period of time, at which point you usually get your initial investment back.
Please note: A bond is an investment and is therefore not totally risk-free. Corporations and countries can go bankrupt or not have enough capital to repay the bond. If the bond is issued in a currency other than EUR, you risk losing some of the money you have invested if the exchange rate moves unfavourably.