A mixer is a module that combines many sound sources.
Each sound source is in a track. Each track has an input, a gain module with a slider, and a visual meter of amplitude. Some mixers also include filters, amplitude modifiers, panning, or other effects as options per track. Some mixers allow us to change their parameters over time. Even more, some mixers allow for advanced signal routing functions. The purpose of these additional features is to balance the sound sources to create a pleasant or interesting overall result.
A mixer can be any of analog hardware, digital hardware, or software device. Within a digital audio workstation, the mixer is usually software. Sometimes mixers are also known as mixing consoles.