This is the 3347th most frequent French word.
épais
Thick.
Here, "épaisse" describes the physical thickness of the book's cover, indicating its material strength or density.
Le livre qu'il lisait avait une couverture épaisse.
The book he was reading had a thick cover.
In this sentence, "épaisse" refers to the consistency of the soup, implying it is dense and possibly needs to be diluted.
Cette soupe est trop épaisse pour être consommée telle quelle.
This soup is too thick to be consumed as is.
Here, "épaisse" is used metaphorically to describe the density of the forest, indicating closely packed elements such as trees and foliage.