What is it about urine that makes a dead person be punished for it in his/her grave? Is it that if he does not clean himself from urine, he is punished in the grave? Could you please explain this in some detail and I do not find the matter embracing at all since it is a serious matter.

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

It is proved in the Sunnah that anyone who does not clean himself from urine will receive torment in his grave. Ibn ‘Abbas narrated that the Prophet once passed by two graves and those two persons (in the graves) were being tortured. He said, “They are being tortured not for a great thing (to avoid). One of them never saved himself from being soiled with his urine, while the other was going about with calumnies (to make enmity between friends). He then took a green leaf of a date-palm tree, split it into two pieces, and fixed one on each grave. The people said, “O Allah's Apostle! Why have you done so?” He replied, “I hope that their punishment may be lessened till they (the leaves) become dry.”

In the above Hadeeth, it is clearly defined that the one who does not take precaution against urine deserves the torment of the grave. Imam Ibn Khuzaymah reported in his Saheeh on the authority of Abu Hurayrah that the said: “Urination is the main cause of punishment in the grave”; i.e., for not having kept away from urine to touch one's body or clothes. One reason for this torment might be that not cleaning oneself from urine causes ritual impurity, and in the state of impurity, the prayer of the person is not valid, so it causes the annulment of the prayers which is also another great sin.

Allah knows best.