There are two types of analog audio signals that we deal with in professional audio: unbalanced audio and balanced audio.
Balanced audio is the standard format for professional audio systems; unbalanced is used in consumer audio systems and in most musical instruments.
Unbalanced audio is delivered through one wire, containing the signal, surrounded by a shield which helps to protect the signal wire from outside interference. Despite the shield, the audio signal is very susceptible to interference from other sources of electromagnetic energy. Power cables running near the audio cables can generate a 60Hz hum in unbalance audio lines, and they are more susceptible to RF interference from things like cell towers and radio stations. Because of this, unbalanced audio cables should be kept as short as possible (10 feet or less is a good rule of thumb). The longer the cable, the more opportunity for interference to find its way into your audio signal.
Balanced audio provides two wires for the audio signal surrounded by a shield and is designed for long signal runs. One of the signal wires carries half the audio signal; the other signal wire carries the inverse of the other half of the audio signal. So, if the audio signal at a point in time is at 1, the first wire has a value of +0.5 on it, and the second wire has the inverse, or -0.5, on it. These two wires are wrapped tightly around each other through the length of the cable. Because of this wrapping, any interference that affects one of the signal wires will affect the other signal wire in exactly the same way, and in the same polarity.
When the balance audio signal enters a piece of audio equipment like an audio mixer, the equipment essentially subtracts the value of the signal on the second wire from the value on the first wire to get the original signal value. (It’s more complicated than that, but that’s the mathematical gist of the process.) In the previous example, if at a point in time the original signal is 1, and if 0.1 amount of noise gets through the shield and introduced into the audio signal, then the receiving end of the signal sees 0.6 and -0.4 on the two wires. 0.6 - -0.4 gives you 1 – exactly the original signal value without the noise. I.e., the noise gets cancelled out.
This is why you run your musical instruments like guitars and keyboards through direct boxes. A direct box converts an unbalanced signal into a balanced signal, enabling you to DIRECTly connect your guitar or keyboard into your professional audio system. Thus, you keep the potentially noisy signal run very short, and use the balanced lines to get you from your stage to your audio mixer.