In Death Note MU refers to Nothingness. However, since, in this subject, there is always room for speculation, and not even Shinigami can come back from the dead to tell us what MU is, it is possible that MU is a neutral respite for souls before moving on to another life (if you believe in rebirth). The fact that it offers rest regardless of previous deeds makes it neither a Heaven nor a Hell, as no sanctions are offered.