This is the 8352nd most frequent Polish word.
pełen
"Pełen" means "full" or "filled" in English.
'pełen' is used to refer to the physical state of something being completely filled with a content, in this case apples in the box.
Skrzynka była pełna jabłek.
The box was full of apples.
'pełen' is used as an adjective describing the park to indicate that it was completely occupied or abundant with children.
Park był pełen dzieci.
The park was full of children.
'pełen' is used metaphorically to express a state of emotional abundance, describing the heart as possessing love entirely.