A botnet is a group of computers that have been taken over by bad people without the computer owner knowing. Just like how a toy robot can be controlled by a remote, these bad people can control all the computers in the botnet to do things without the real owner knowing.
The bad people do this by sending a special program to the computer that they want to take control of. Once the program is installed, the bad people can start controlling the computer with it. The computer can then be made to do all kinds of things like sending spam emails, attacking other computers, or stealing data.
It is really bad because not only can the owner of the computer lose control of it, but the computer can also be slowed down or even stop working. Imagine if someone took the remote control for all of your toys away, it would be really annoying and not fun anymore.
It is important to have good security on your computer and be careful with what you click on or download because then bad people can not get in and take control of your computer. Just like we have to lock our doors and be careful around strangers, we need to do the same thing with our computers to stay safe.