This is the 11879th most frequent Russian word.
заполнен
"Заполнен" means "filled" or "full" in English.
Here, 'заполнен' is used as a past participle to describe the diary being fully occupied with entries.
Мой дневник полностью заполнен.
My diary is completely filled up.
In this sentence, 'заполнен' suggests that the form needs to be completed with information.
Форма для регистрации должна быть заполнена.
The registration form must be filled out.
Here, 'заполнен' refers to the space of the room being occupied.