A mixdown is a collapsed version of data in the Motion Mixer. In general, a mixdown contains the same data as the raw mix, with one difference. During the process of computing a mixdown, any transitions involving planted feet are corrected to prevent the feet from sliding.
After a mixdown is computed, it is placed on its own track in the Mixer. You can turn the mixdown on and off by clicking it. When the mixdown is turned on, the biped performs the motions in the mixdown; when it is turned off, the biped does the motions in the raw mix. See Exporting Animation to the Biped.