The Iron Throne is the literal and figurative seat of power of the fictional monarchy of Westeros in the Song of Ice and Fire series of fantasy novels by George R.R. Martin, upon which the kings [their hands*, and one queen**, Cersei Lannister (played by Lena Headey in the show)] of the Andals and the First Men sit, and is allegedly forged from 1,000† swords surrendered to Aegon Targaryen (aka Aegon the Conqueror) during the War of Conquest, and fused together by dragon’s fire.
Martin has rarely been satisfied with representations of the Iron Throne in books, games or even the wildly successful HBO TV series, and maintains that only its depiction by French concept artist/illustrator Marc Simonetti in the Song of Ice and Fire companion book The World of Ice and Fire (Bantam Books, 2014) is “absolutely right”.
*Closest appointed advisor, and in absentia, proxy to the ruling monarch
**Rhaenyra Targaryen also had a stint on the Iron Throne before being deposed and executed by her half-brother Aegon II, who then declared her brief and unpopular rule unofficial
†Actual number of the swords contained in structure is less than two hundred (Game of Thrones season 3, episode 6, The Climb)