As these guys have said, there are two different things they can refer to as "depth".
Gloss is probably the wider interpretation of "depth", as the reflections you see on the surface seem like you are looking into a mirror, and seeing the image of the world behind you.
"Depth" is also used to describe the distance between the top surface reflections.... and the light bouncing off of the deeper base color... especially a highly reflective metallic or pearl. Your eye can see the distance between the two types of reflection.
You can add even more "richness" with the affect of the light reflecting back, and illuminating a tinted transparent color, producing a rich "lit from within" appearance. ...but you still need that original high gloss to make it look it's best!