Literally: the sun is dead
This phrase references a cosmic event known as a solar eclipse. Last Sunday was the most recent sun death, 18 years after the previous visible eclipse.
The first word is the sun, and the second word has a stem that could mean falling, but in this form it means death.
In the Navajo sense, the sun dies and is reborn again as the eclipse passes. Many Navajo observed the eclipse by staying inside and not eating food for either the duration of the event or through the night.
For the word for moon, see tł’éé’honaa’éí.