This is the 8293rd most frequent French word.
willem
"Willem" is not a French word; it is a Dutch given name equivalent to "William" in English.
Here 'Willem' is used as a proper noun, specifically a given name.
Willem est un prénom néerlandais très courant.
Willem is a very common Dutch name.
In this usage, 'Willem' specifies the name of the speaker's friend, functioning as a specific identifier.
Mon ami Willem est venu pour dîner hier soir.
My friend Willem came for dinner last night.
Here 'Willem' is used within the title of an artwork, representing the subject of the portrait.