This is the 398th most frequent Spanish word.
estaban
"Estaban" means "they were" or "you (plural) were" in English.
The word 'estaban' here is used as an imperfect tense verb indicating a past continuous state of being happy.
Ellos estaban felices ayer cuando recibieron la noticia.
They were happy yesterday when they received the news.
In this context, 'estaban' indicates a past placement or location of the subject (the books).
Los libros estaban sobre la mesa antes de que los moviera.
The books were on the table before I moved them.
Here, 'estaban' describes a past continuous condition of the lights being off.