Color depth refers to the number of unique colors that a sensor can faithfully capture. Cameras with better color depth will demonstrate smoother transitions from one color to the next, and avoid "banding" in areas with subtle shading. (Blue skies are an example of where you might see color banding.)
More color depth is generally better, but you'll struggle to notice a difference between two cameras with less than 1 bit of difference. Color depth above 22 bits is considered excellent.
Want to learn more? Head over to our full glossary entry on color depth
Color depth test data courtesy of DxO Mark.
Z7 test data on DxO Mark
D5000 test data on DxO Mark