In أَسْبِغُوا الْوُضُوءَ, أَسْبِغُوا is a plural imperative: “all of you complete thoroughly.” الْوُضُوءَ is its object. The command teaches the group not to neglect the prescribed areas of washing.
الْأَعْقَابُ means the heels, the rear portions of the feet. The report specifically states that water had not reached them; casually splashing or assuming that all areas are wet is insufficient. A person should pay attention to portions of the feet that can easily be missed when rushing.