Frequency Domain
The Frequency Domain is simply the representation of audio in terms of the frequencies that are present at any given moment. It is often shown in graphical form from 20 to 20,000 Hz, the range of human hearing, with the frequency increasing over the x axis while the y axis represents amplitude. This contrasts with the Time Domain, where the x axis represents time.
Looking at the frequency domain can display the fundamental and its overtones of a particular instrument as well as demonstrate the range of an instrument. It is very helpful for mixing as well identifying noisy or static frequency ranges that can be filtered to improve SNR and clarity for an instrument.