What do you understand by Miller indices of a crystal plane?

Miller Indices are a symbolic vector representation for the orientation of an atomic plane in a crystal lattice and are defined as reciprocals of the fractional intercepts which plane makes with the crystallographic axes.

In an orthogonal coordinate system, interplaner distance d_hkl is given by:

Where h, k, l are the Miller indices of planes and a, b, c are dimensions of the unit cell.

Now for a cubic system, a = b =c

Hence, d_hkl= a / [h²+ k²+ l²]^1/2