An ADC, or Analog to Digital Converter is an integrated circuit that converts analog signals to digital ones.
First, the signal is sampled. This is performed by reading the voltage of the signal over time. Then, the continuous voltage is quantified and stored as a discrete number. ‘These now-binary values can then be sent to another part of the system to be read converted back into analog signals to drive a speaker.
The two most important aspects of an ADC are its resolution (how many levels of different voltage it can detect) and sampling rate (how many times per second it can read the voltage).