I assume fs means sampling frequency . In case it is being applied while encoding sound files, we can presume it has something to do with compression or encoding. Now typical (as per research) human hearing range is 20-20000 hz. To approximate an analog signal in digital, we need to sample at twice the rate - which is 40000 hz (as per Nyquist). Generally most of the amplifier output curve is bell shaped, so having sharp cut off at 40000 hz will hurt fidelity & we need some protective overage. Sony choose 44100 rate while creating compact disc. That's how the tradition is born,