This is the 13521st most frequent Polish word.
pełny
"Pełny" means "full" in English.
The word 'pełny' here is used as an adjective describing the room being filled with people.
Pokój jest pełny ludzi.
The room is full of people.
In this sentence, 'pełna' (feminine form of 'pełny') describes the answer being rich in details.
Jego odpowiedź była pełna szczegółów.
His answer was full of details.
Here, 'pełnię' (a noun form derived from 'pełny') signifies a state of being complete or fulfilled.