What does Isaiah 14:15 mean?

The greatest sin of the king of Babylon, from the Lord's perspective, was his enormous pride and arrogance. He imagined himself to be self-made in his power over Babylon and the world. Babylon's king believed his rise in power would continue until he sat among the gods like the Most High (Isaiah 14:13–14). He never understood the reality that all human power is given and taken by the Lord God of Israel. He was nothing more than a puppet.

Death is the ultimate cure for arrogance. It is the final leveler of all humanity. The king who imagined himself a god, was instead brought down to Sheol (Isaiah 14:9). This was the place thought to be the destination for all the dead by the peoples of the ancient Near East. Isaiah is not attempting to offer an analytical, precise theology of the afterlife in this poem. He is using poetry and song to mock the would-be god by describing his final resting place in the pit of Sheol. The king's end destination was no different from those he considered lesser than himself. In fact, the king was not even given a prime spot in Sheol. He languishes in the "far reaches" of the pit. He has been brought lower than the low.