Hey little buddy, have you ever played with a toy that had a little problem that made it not work the way it was supposed to? Maybe it didn't move the way it was supposed to, or maybe it wouldn't make any noise when you pressed the button. Just like toys, computers can also have problems like this.
When a computer program has a little problem that makes it not work the way it's supposed to, we call that a "bug." Bugs can happen because when people make computer programs, they might accidentally make a mistake somewhere in the code. Just like how you might accidentally build your tower too tall and it falls over.
Sometimes, a bug might not be a big deal and the program might keep working just fine. But other times, the bug can cause the program to not work at all, or even cause other problems in the computer.
Luckily, when bugs are found, programmers can work to fix them by figuring out what went wrong and then rewriting the code to make it work better. Just like when you fix your toys by trying a few different things until it works again.
So, a computer bug is just like a little problem in a computer program that can cause it to not work properly. But don't worry, there are smart people out there working hard to fix them when they happen.