When an audio signal is digitized it is converted into discrete digital numbers (0's and 1's) that make up the digital file. A "Bit" is a single piece of information assigned a value of 0 or 1.
The most common method for encoding an analog signal into digital data is pulse-code modulation (PCM).
PCM (Pulse Code Modulation) is based on a series of measures ("samples") of the amplitude of the analog signal.