A Color Rendering Index (CRI) is a quantitative metric of the ability of an artificial light source (i.e. LED, Fluorescent, Halogen, Incandescent, etc.) to accurately reveal the colors of the subject being lighted in comparison to a natural light source, the sun.
Sunlight, the standard for CRI, is considered to most accurately reveal the colors of a subject.
A CRI of 90 means that the artificial light source is replicating roughly 90% of the visible color spectrum that the sun would produce on the same color.
It is important to note that CRI is independent of color temperature (CCT).
Applications that may require high 90+ CRI include art gallery lighting, lighting for museums, and retail lighting.
CRI is not a measure of brightness, but how sharp colors would appear under the given light source.
In any given space, the CRI of the light source could have a profound effect on how the paint looks, how the furniture look, and how the people look and feel.