Miller indices are the reciprocals of the intercepts, made by the plane on the crystallographic axes, when reduced to smallest integers.

Miller indices are used to designate the planes in a crystal lattice.

Intecepts on x, y and z axes :-      1, 2, infinity
The reciprocals of intercepts :-      1/1, 1/2, 1/infinity
i.e.      1/1, 1/2, 0
Reciprocals are reduced to smallest set of integers by taking LCM :-      2, 1, 0
So, miller indices :-       (2,1,0)