Explanation of the hadith
The Prophet (may Allah's peace and blessings be upon him) warns against negligence in the performance of ablution and encourages us to perform it perfectly and completely. As the back of the foot (the heel bone) is normally somewhere that water does not reach, it leads to a deficiency in ablution and therefore in prayer, so the Prophet (may Allah's peace and blessings be upon him) informs us that the heel bones, along with the one who is careless about including them in his Shariah-required purification, will be targeted by torment in the Hereafter.