50% dmg

Hi, playing wow for the first time and i was wondering whyu my ranged damage stats are reduced by 50% somtimes, its red and says 50% dmg but no explenation, is it something to do with using a staff as meele wepon maybe. pls help cant find anything on google

Aspect of the Viper reduces your damage by 50% but allows you to regenerate mana based on your Attack Power.