وَيَحْمِلُ عَرْشَ رَبِّكَ فَوْقَهُمْ يَوْمَئِذٍ ثَمَانِيَةٌ (...And on that Day, the Throne of your Lord will be carried above them by eight [ angels ]....69:17). In other words, on the Day of Judgment, eight angels will bear the Throne of the Rahman above their heads. According to some of the Prophetic narratives, before the Judgment Day this task will be carried out by four angels. On the Judgment Day, four more angels will be added to carry out the task.
As to what is the Throne of the Rahman, what is its reality, its form and shape, how the angels carry it - are transcendental matters and are all beyond human intellect. Allah is beyond time, space and matter, and therefore we are not permitted to pursue these matters pertaining to Him to determine its exact meaning. The noble Companions, their pupils and the righteous elders took the attitude that they simply believed in all transcendental matters and never questioned about them. Whatever Allah means by them is the Truth. Their nature and reality are unknown.