Okay kiddo, so a kill file is like a list that tells your computer certain things you don't want to see online.
Let's say you go to a website and there's a person or a topic that you don't like. You can create a kill file and add their name or the topic to it, and then your computer will automatically hide any posts or comments about them.
It's kind of like putting your hands over your ears when you hear something you don't want to listen to.
The kill file helps you to avoid seeing things that make you upset, so you can have a better experience online.