What is mcd?

Candeira

(Candela) I = F/Ω luminous flux emitted by a light source in a unit solid angle in a specified direction.

brightness cd/m? The brightness of the light-emitting surface refers to the ratio of the luminous intensity of the light-emitting surface in the specified direction to the area of the light-emitting surface perpendicular to the specified direction.

1cd is 1 Candeira, mcd is one thousandth of cd, just as mm (millimeter) is one thousandth of m (meter), of course, 4 is brighter.