The basic idea of a CRO: Sound energy is transformed into an electrical signal at the microphone. The size of the voltage induced at the microphone is a function of the air pressure of the air striking the microphone. That pressure differential changes the voltage to a higher or lower value as it passes into the CRO. The voltage input registers on screen as a waveform.
Uses include the ability to view sound waves. (Basically what it is used for)