In its simplest form an audio mixer combines several incoming signals into a single output signal. This cannot be achieved simply by connecting all the incoming signals in parallel and then feeding them into a single input because they may influence each other. The signals need to be isolated from each other. Individual control of at least the level of each signal is also required.
In practice, mixers also do rather more things than simply mix. They can provide phantom power for capacitor microphones (see ‘The capacitor or condenser microphone’, Chapter 3); pan control (whereby each signal can be placed in any desired position in a stereo image); filtering and equalisation; routing facilities; and monitoring facilities, whereby ...