Mixing is the proper blending of all the audio elements/tracks in a project/song to create a final audio product/mix. The mix engineer adjusts each track to get the proper balance and blend of the various sounds. Equalization (tone control) is used to adjust the tonal quality of each sound. Other signal processing devices (reverb, delay, compressors, noise gates, etc.) are used to further shape the sound. Automation is used to memorize adjustments within the song. 

The "original final mix" are recorded in a 2 track (stereo) format such as CD, 24bit files or in 5.1 surround. When all mixing is completed on a project, it is ready for mastering.