This is the 2270th most frequent Polish word.
studenci
"Students" (plural form).
Here, 'studenci' is the subject of the sentence, referring to a group of individuals who are engaged in academic studies at a university.
Studenci uczą się na uniwersytecie.
Students study at the university.
In this sentence, 'studentów' is the plural genitive form of 'studenci', used as the object being observed.
Widziałem studentów w bibliotece.
I saw students in the library.
Here, 'tych studentów' refers to specific students in the accusative plural form, as they are the object of the action of knowing.