"обнулят" is the future form of "обнулить" (perfective) "to make equal to zero, to set to zero" from "нуль" (or "ноль") - zero, nil, 0.
BTW, the imperfective pair to "обнулить" is "обнулять".
Usually it is used in expressions like "обнулить счётчик" (to set a meter or a counter to zero, to reset it).