What does it means?
It means that some sample in the light cache has either a negative or invalid (like infinity etc) RGB component. This is similar to the “Material returned overbright or invalid color” warning that you may get.
Best regards,
Vlado
thank’s 
ok, but so what? what can we do to avoid it or we can go ahead without worry with those warnings? and i would like to know how to let a negative rgb value positive, because this is one of the most commons messages we get around
It depends on what is causing the error. If it is a texture map, turning on its Clamp option (in the Output rollout) might help. If it is a shader (e.g. the error occurs even without any textures), then changing the shader parameters may help.
Best regards,
Vlado