Ground Loop Isolator
If you hear a buzzing or humming noise when your computer and mixer/amp are connected then it's usually a ground loop. You haven't done anything wrong so don't panic! You can get a ground loop at any time as it's down to how the mains earths are wired in the building.
Breaking the earth connection between your sound card and the mixer/amp will cure this problem. That is exactly what a ground loop isolator does.
They don't use batteries so just fit and forget. Most DJ's have them permanently attached to the back of the mixer so you can't forget to use them.
Each isolator has stereo sockets on one side and stereo plugs on the other so just connect the line from your soundcard to the isolator then plug the isolator into the mixer/amp.
You need one isolator for EACH stereo line from your computer to the mixer/amp.