Mixing is the process of combining all the elements of a song into the final 2 channel (stereo) audio. During the mixdown you adjust things like the volume/levels of sounds, EQ, compression, effects, etc. A lot of important decisions about how the audio is processed are made during this stage which is why a bad mix job can ruin otherwise great work. Mixing down is not a something you want to rush through. It's truely a creative art form for people who know what they're doing.