In audio, source refers to (you guessed it) the source of the audio. This would seem to be obvious, but in the context of how it applies to audio and video equipment it sometimes isn’t obvious. It mostly applies to how signals are monitored. You can determine whether you monitor the signal coming in from the source or at some later point inside a device. For example, the source button on a cassette deck might switch you between monitoring the signal coming off the tape to the signal being applied at its inputs (the source).