We're talking about a set of points already in a plane, so consider the x-y plane, in which the equations y = 5 and y = -3 correspond to vertical lines. All the points equidistant from both lines would fall on the vertical line right in the middle of y = 5 and y = -3, which we can get by taking the average:
(5 + (-3)) / 2 = 2 / 2 = 1
So this locus is the single vertical line y = 1.