The ability to blend audio signals together in various proportions and in changing amounts over time is an essential technique for the manipulation of audio. The mixer fulfills this function. 
A mixer will typically have three main functions:
A mixer must have some provision for inputting two or more signals and adjusting their respective levels. 
Some form of equalization or filtering of the inputs is usually provided, as well as panning between output channels. Also some form of auxiliary send is usually provided so that external processing of the signals can be done. 
The output of the mixer consists of one or more channels or busses. These are sent to amplifiers for playback and monitoring or to recording devices. 
cables and connectors
input/output stage