This is the 727th most frequent Spanish word.
aquí
"Here."
Here, 'aquí' is used to indicate the location where the book is found, denoting proximity to the speaker.
Aquí está tu libro.
Here is your book.
In this sentence, 'aquí' specifies the location of 'here' directly associated with the presence of both persons.
¿Quién está aquí conmigo?
Who is here with me?
The word 'aquí' here functions to point out the destination towards which the action should be directed, close to the speaker.