Usually a preview of 3D visualization, something like a draft. However, clay render does not contain any textures or materials (unless specified otherwise).

Most of the time, you would be seeing the scene and all the items in it with just a plain white or grey material applied. Clay render is sometimes a quicker way of approving the light, composition or product positioning since the materials and their physical properties do not have to be computed and accounted for in the preview render, which sometimes might save a bit of time when client is in a hurry.

Since clay renders do not contain materials, any light cast or bounce or perhaps any major reflection and refraction issues might not be present and thus we strongly recommend going with fully textured drafts instead of clay ones.