This is the 417th most frequent Icelandic word.
eystra
"Eystra" means "eastern" or "to the east" in English.
Eystra is used here as an adverb describing a location relative to the east side.
Fjallið verður fallegast við sólarlag eystra.
The mountain is most beautiful at sunset in the east.
Eystra is used as an adjective modifying 'hæðinni' (the hill), specifying its eastern position.
Þar fundum við forsæluna á hæðinni eystra.
There we found the cool shade on the eastern hill.
Eystra acts as an adverb indicating the origin of the light from an eastern direction.