‘Sit there silently, and get into darkness, O daughter of the Chaldeans,
For you will no more be called ‘the Lady of the kingdoms'.'
Here Babylon is depicted as being degraded and becoming like a lowly ladies' maid or servant who is dismissed to sit quietly in the darkness, away from the lighting, until bidden, and is not allowed the luxury of sharing the bright lighting. She is excluded from the inner circle. Thus Babylon is no longer to be the proud ‘Lady of the kingdoms', feted by all. She is to lose her position, her luxuries and her privilege. She is to be humiliated and to become a serving woman. And in the end her silence and darkness will be permanent.
Others see ‘silence' as signifying loss of authority and therefore of a right to speak, with darkness possibly indicating imprisonment.
(Some see ‘darkness' as always referring to Babylon in this section (see Isaiah 42:7; Isaiah 45:19) but it is difficult to see how that is so here. It could be seen as indicating exile, away from the land of light, but why should Babylon be the land of light? It is far better to see it as a general expression of lack of what is good or as lack of light).