This is the 10269th most frequent Polish word.
cennej
The word "cennej" in Polish means "valuable" or "precious" (feminine singular genitive or dative form).
Here, "cennej" is used as an adjective describing "wiedzy" (knowledge), in its singular genitive feminine form.
Otrzymałem książkę pełną cennej wiedzy.
I received a book full of valuable knowledge.
In this sentence, "cennej" is an adjective modifying "rady" (advice), which is in the singular genitive feminine form.
Skorzystaliśmy z cennej rady ekspertów.
We benefited from the valuable advice of experts.
This example employs "cennej" as an adjective for "mikstury" (potion), in its singular genitive feminine form, showing the value attributed to the potion.