This is the 10114th most frequent Polish word.
biologii
"Biologii" means "biology" (genitive case) in Polish.
The word "biologii" is used here in the context of academic studies, following the preposition "z" indicating the field of study or topic related to graduation.
Ukończyłem studia z biologii na uniwersytecie.
I graduated in biology from university.
Here, "biologii" is used in a genitive case construction with "do" to indicate the subject or topic of the textbook.
Podręcznik do biologii znajduje się na półce.
The biology textbook is on the shelf.
In this sentence, "biologii" appears after the preposition "na" to describe the type of lesson conducted.