Frequency-shift keying
Frequency-shift keying, or FSK for short, is a way to convert binary data into sound and back again, in order to transmit or store it using technology designed for audio signals.
Say you have a few kilobytes of data you want to transmit over a phone line, or store on a cassette tape. Your first impulse might be to simply send the binary signal directly, so it's high for each binary one and low for each binary zero.
Alas, this is slightly oversimplistic: anything designed for audio tends to have a DC blocking capacitor — basically used as a highpass filter — to ensure the whole signal isn't accidentally offset, known as a DC bias. This automatically removes things such as wayward CVs that crept into a synthesiser's audio output, and unnecessary sub-bass that's so sub it's inaudible, saving phones and tapes from wasting energy reproducing a part of the signal you can't hear, probably wouldn't want to hear even if you could, and that might degrade the quality of the part you can hear. But it's a problem if you want to send a bunch of zeroes or ones in a row. You need to keep the signal moving at all times, in the audio range, regardless of what those bits happen to be.
A simple solution is to essentially use frequency modulation, in a very basic, binary-only way: send a steady tone, say 1.3 kHz, to represent a string of zeroes, and then change it (modulate it) to, say, 2.1 kHz whenever there's a binary one to transmit. That way, regardless of how many zeroes or ones you have in a row, you're still transmitting or storing an audible signal that can be safely received or retrieved.
A circuit that can modulate data into an audible signal to transmit or store it, and demodulate it back again to receive or retrieve it, is known as a modulator/demodulator... or modem for short.
As simple as it is, frequency-shift keying is pretty reliable. It enabled dial-up BBSes and Internet access, tape sync, and software stored on cassette tape.
Modulation: FM synthesis | Frequency-shift keying | Frequency shifter