Mixing or the mixdown process involves combining the final individual audio tracks (multitrack) into a final stereo (left and right) mix surround mix(5.1) or mono mix.  Individual tracks are carefully blended using tools such as equalization to shape tone, compression to dynamically balance where tracks sit in the balance, reverb and delay to add overall ambiance to the songs , mix.  The engineer skillfully blends these tracks to the artist's/band's and or producer's taste.  Getting the 'perfect' balance requires trial and error, careful listening, and sometimes remixing until the mix is energetic and reflects the musical passion of the performance.  The completed mixes are the put in an order for the final album, before going to the final stage ....mastering.