"Missing detail texture in Terrain, degraded loading performance"

What does this warning mean? I can't figure out how to fix it. I am new to Unity and it seems that I traded the "Fultrum"error for this warning. I have texture in my scene and have never gotten this warning until now. What can I do to fix it?

This warning means that one of the detail textures in the terrain is missing. In the terrain inspector, check the "Paint texture" tool, and see if there are any empty slots in the texture list. Maybe you deleted one of the textures referenced there.