This is the 2427th most frequent Russian word.
погибшего
"Погибшего" means "of the deceased" or "of the fallen."
Here, "погибшего" is used as an adjective to describe the noun "солдата" (soldier), specifically referring to a soldier who has died tragically.
Цветы были положены на могилу погибшего солдата.
Flowers were placed on the grave of the fallen soldier.
In this sentence, "погибших" is used as a noun (a substantive participle) in the genitive plural form to describe individuals who died in the fire.
Список погибших в пожаре был опубликован в газете.
A list of those who perished in the fire was published in the newspaper.
Here, "погибшего" functions as a genitive singular form of a substantive participle, referring to a specific individual who has passed away.