This is the 1048th most frequent Portuguese word.
lá
"lá" means "there."
In this sentence, 'lá' is used to indicate a distant location where the speaker was yesterday.
Eu estive lá ontem para visitar um amigo.
I was there yesterday to visit a friend.
'Lá' is used here to point out the location of an object, meaning 'there' as the position of the book.
Lá está o livro que você estava procurando.
There is the book you were looking for.
In this sentence, 'lá' is used to refer to a specific remembered or imagined location the speaker talks about.