These lines refer to Shah Jahan and his beloved wife Mumtaz Mahal.
Taj Mahal was built by Emperor Shah Jahan for his wife who passed away too quickly for her age. Taj Mahal has a heavenly charm, but we have to still remember that even though it's beautiful, it's like salt being rubbed on a wound. It hurts more when you look at it and remember why it was build.
All the beauty of Taj Mahal means nothing in comparison to the emperor's loss.