A bond event is like a little party that happens whenever you buy or sell a bond. A bond is like a fancy IOU that a company or government gives you when you lend them money. When you buy a bond, it's like you're giving them a loan, and they promise to pay you back with interest after a certain amount of time.
But sometimes things happen that can affect the bond party. For example, if the company or government that owes you the bond money is having a hard time making payments, they might have something called a "default," which means they can't pay you back. That would be a bad bond event.
On the other hand, there can be good bond events, too. For example, if the company or government that owes you the bond money is doing really well, they might decide to pay you back early, which is called a "call," or they might give you more money than you expected, which is called a "coupon."
So, basically, a bond event is just a fancy way of talking about things that can happen when you lend someone money by buying a bond, whether they're good or bad.