An audio signal is the direction audio is traveling through a sound system. So in a basic situation, this could simply be the audio signal from your smartphone to your ears. It's one connection, headphones, and the audio signal travels from the smartphone through the cable to your ears. When sound systems and gear for live performances begin to mount up, quickly and accurately assessing an audio signal is paramount. The best thing to do to figure it out is to work slowly and methodically. Drawing a diagram out on paper isn't a terrible idea if you need to tear down and set up DJ equipment often.