A mixer or mixing table is part of a PA (= public address) system. This is a sound installation that amplifies music for an audience.
The mixer aims to be able to process a large number of inputs into a stereo sound that can be sent to the amplifiers or to the active speakers. The most important thing here is the way the system converts and processes channel by channel. Good PA systems ensure that as little as possible of what enters the system is lost or distorted. Certain incoming signals are deliberately distorted (such as equalizing, compression, adding reverb, etc.) to make the whole more beautiful or pleasant (think of a feedback (= pervasive whistle) that is filtered out).