This is the 13513th most frequent Polish word.
pełnego
"Pełnego" means "full" or "complete" in English.
In this sentence, 'pełnego' is used as an adjective describing the noun 'książkę', in genitive case matching the object of the verb.
Otworzył książkę pełnego wiedzy.
He opened the book full of knowledge.
Here, 'pełnego' works as an adjective describing 'szczęścia', conveying the possession or abundance of 'happiness' in genitive case.
To był dzień pełnego szczęścia.
It was a day full of happiness.
In this instance, 'pełnego' modifies 'chleba', indicating the bag is filled with 'bread', agreeing in case and number.