This is the 672nd most frequent Polish word.
fundacji
"Fundacji" is the genitive or dative form of "fundacja," meaning "foundation" (an organization or institution).
Here, 'fundacji' refers to the foundation whose address is being discussed. It is in the genitive case to signify possession (of this foundation).
Adres tej fundacji można znaleźć na stronie internetowej.
The address of this foundation can be found on the website.
In this sentence, 'fundacji' refers to an organization (foundation) dedicated to a specific charitable cause, in this case, helping children, and it is in the dative case to indicate the recipient of the action (donation).
Książki zostały podarowane do fundacji pomagającej dzieciom.
Books were donated to the foundation helping children.
In this case, 'fundacji' represents the source of the materials being used, and it is in the genitive case due to the relationship expressed by 'z' indicating material origin.