This is the 87th most frequent French word.
étaient
"Étaient" means "were" (imperfect tense of "to be").
Here, "étaient" expresses a state of being in the past (imperative mood) for a feminine-plural subject, highlighting their happiness.
Elles étaient très heureuses de voir leurs amis.
They were very happy to see their friends.
In this context, "étaient" describes the appearance or quality state of the books in the past, using the imperfect to indicate a continuous or general past description.
Les livres sur l'étagère étaient anciens et précieux.
The books on the shelf were old and valuable.
This sentence uses "étaient" as the third-person plural imperfect tense of "être" to describe an ongoing past activity where the children are playing.