Well in short, the term Mixing may be referred to either to the final stage in the production of a audio program— typically a song, but can also relate to a radio show, TV show, movie, commercial, webcast, or any other audio program material. In case of mix related to Motion Pictures it can also be known as Re-Recording Mixer, one who is responsible for overall flow of audio with respect to desired mood of the movie.
In this final stage, all of the audio elements are combined to create the final version of the audio program in its intended format. Typically, that format is a stereo audio file, but it might be surround sound (requiring several files) or even mono, and it may be analog tape or some other storage media.
You are no doubt aware that most music is created from multiple sources that occupy many tracks in a typical DAW. All of these elements need to be set for level, panning, and processing (EQ, compression, reverb, etc.). Mixing, thus, is creating the final placement and sound of each element over the course of the musical time line. While mixing generally the creative imagination serve the mixer’s sonic vision of the final audio.