This is the 6712th most frequent Dutch word.
willems
"Willems" is a Dutch surname, meaning "son of Willem" (equivalent to "Williams" in English).
In this sentence, 'Willems' is used as a possessive form of the name Willem, indicating ownership of the book.
Willems boek ligt op de tafel.
Willem's book is on the table.
Here, 'Willems' is part of a proper noun, specifically the name of a park, demonstrating its use within a compound noun.
We bezochten het Willemspark gisteren.
We visited the Willemspark yesterday.
In this instance, 'Willems' is again used in a proper noun context as part of a street name.