This is the 1603rd most frequent German word.
quer
"Quer" means "across" or "crosswise" in English.
In this sentence, 'quer' is used as an adverb to describe the position or orientation of the book—positioned across or at an angle.
Er legte das Buch quer auf den Tisch.
He placed the book crossways on the table.
Here, 'quer' as part of the verb 'überqueren' indicates the action of moving across something, such as a street.
Sie überquerte die Straße bei Grün.
She crossed the street when the light was green.
In this case, 'quer' serves to emphasize the direction or trajectory, meaning across in the context of the airplane's flight over the ocean.