What are concealment (al-istitār) and transcendence (at-tanazzuh), what is the difference between them, and how do we reconcile the two narrations mentioned in Sahih al-Bukhari and Sahih Muslim regarding the torment of the two graves?

The Prophet's (peace be upon him) hadith narrates that two men are punished in their graves: one of them did not avoid his urine or guard himself from it, and the other engaged in backbiting (namimah). The authentic narrations' wordings, "لا يستتر" (he does not shield himself), "لا يستبرئ" (he does not clear himself), "لا يتنزه" (he does not keep himself clean), all mean not guarding against or avoiding urine.