A DSL filter is a little tool that helps your internet work better at home. When you use the internet, your computer sends and receives signals through the telephone line. But your telephone line is also used for talking on the phone. This can cause problems for your internet signal because the phone signal also travels through the line.
The DSL filter helps to separate the internet signal from the phone signal so that you don't get any interference or noise. It looks like a little box that you plug into the telephone socket on the wall. Then, you plug your telephone into one socket on the filter and your internet modem into another socket. This way, the filter separates the signals and your internet works much better!
So, think of it like when you are playing with your toys and there are a lot of other noises happening around you, like somebody talking or a dog barking. It can be hard to hear what you're doing, right? Same with your internet signal and phone signals. The DSL filter helps to make sure that only the internet signal gets through without any other sounds interfering!