This is the 5473rd most frequent Russian word.
погибшую
"Погибшую" means "deceased" or "dead" (referring to a female, in accusative case).
The word 'погибшую' is used in the singular feminine accusative case to describe 'рыбу', indicating it is an entity that has died.
Команда нашла погибшую рыбу в пруду.
The team found a dead fish in the pond.
Here, 'погибшую' describes 'героину', denoting the feminine singular accusative case of someone who perished in the war.
Они вспомнили погибшую на войне героину.
They remembered the heroine who died in the war.
In this sentence, 'погибшую' is applied in the feminine singular accusative case to refer to the 'молодую девушку', specifying someone who has lost their life.