This is the 7991st most frequent Dutch word.
ertussen
"Ertussen" means "in between" or "among them" in English.
Here, 'ertussen' refers to inserting oneself into a group or crowd.
Ik probeerde me ertussen te wurmen in de rij.
I tried to squeeze between them in the line.
In this context, 'ertussen' indicates the place of the secret note, being between other items (pages).
Er zat een geheim briefje ertussen in het boek.
There was a secret note between the pages of the book.
Here, 'ertussen' refers to becoming part of a group for a specific purpose, like traveling together.