Light entering a surface, bouncing around inside it and coming out somewhere else — the reason marble, skin, wax and milk look soft and glowing.
In translucent materials, light travels some distance beneath the surface before re-emerging, so each point's colour is blended with its surroundings and shadow edges soften. The depth depends on how strongly the material scatters and absorbs at each wavelength.
It is different from transparency: a subsurface-scattering material does not let you see through it, but it does let light through.
Marble looks luminous beside a painted wall of the same colour because light penetrates and scatters within it.
A flat colour sample cannot represent a translucent material, and measuring one needs care about the aperture and backing.