There are two views, or domains, in which audio must be viewed to make any sense of it.
Firstly, the time domain. This is the most common way of thinking of audio. Sampled audio is a time domain representation of the sound. Viewing sound as time domain information is to view it as a changing amplitude over time.
Secondly audio can also be viewed in the frequency domain. The idea of the frequency domain is that a signal can be represented as a collection of sine and cosine waves. A frequency domain representation of the audio views it as changing amplitude over the frequency spectrum.
These two views can be switched between by using the Fourier Transform.