He's sitting on the Golden Throne. It's an ancient machine that serves several purposes. It has multiple life support mechanisms that keep him from fully dying. He is also fighting to cling to what life he has through sheer willpower and psychic power. Another thing is the Golden Throne allows him to use his psychic power to act as a beacon in the warp to guide ships through the warp like a lighthouse, giving them a point of reference from Holy Terra(Earth). He looks like that partly because He's been on the Throne for 10,000 years in a almost dead state, since one of his traitorous Primarchs Horus mortally wounded him at the end of the Horus Heresy. The Primarchs are his superhuman sons about 10 feet tall and built like a body builder. They were leaders and generals, and lead the space marine legions on the great crusade.
The Emperor on the right in the golden armor, Traitor Primarch Horus on the left, and the Primarch Sanguinius in gold armor slain at Horus's feet. Sanguinius sacrificed himself by engaging Horus before the Emperor got there, landing a blow that opened a hole in Horus's armor that allowed the Emperor to kill Horus despite receiving a mortal wound from Horus.