What s does "Texture Atlas needs textures to have a readable flag set mean"?

What s does “Texture Atlas needs textures to have a readable flag set mean”? this is the error i got when i put three 2d textures into the scripting reference code at http://docs.unity3d.com/Documentation/ScriptReference/Texture2D.PackTextures.html

By default all textures are “locked”, so once loaded into the GPU you can’t read them back via code. You can set read/write enable to true in the Texture import settings to allow read and write operations on the texture. Choose Texture type: advanced.