The animation fix for Hordes of the Underdark is an override that restores the walking and running animations that were used in earlier editions of the game. In addition to aesthetic appeal for some players, this override is reputed to help with certain client crashes related to characters attacking from stealth. (This crash has been notoriously difficult to reproduce consistently, but what is believed to be a fix for it was included in game version 1.69.)
The original fix has not been updated since 2003, so it is not compatible with some features from the more recent patches (such as visible cloaks and some new robes). A more recent version of the fix supports visible cloaks, but not the robes from 1.69.