This is the 575th most frequent Polish word.
pełni
"Pełni" means "fullness" or "fulfills" depending on the context.
Here, 'pełni' is used as an adverb suggesting complete understanding.
W pełni rozumiem twoje zmartwienia.
I fully understand your concerns.
In this context, 'pełni' is used as part of an idiomatic expression indicating the height or full extent of a period, here summer.
Ten miesiąc jest w pełni lata.
This month is in the middle of summer.
Here, 'pełnić' is a verb meaning to perform or fulfill duties or roles.