This is the 111th most frequent Catalan word.
altre
"Altre" means "other" in English.
Here 'altre' is used to mean 'another', suggesting an additional book different from one already in possession or discussed.
Vull comprar un altre llibre.
I want to buy another book.
In this case, 'altra' functions as 'another', indicating a different or additional occasion for going to the park.
Una altra vegada anirem al parc.
Another time we will go to the park.
Here 'altra' translates to 'other', indicating a different excuse besides the one previously mentioned.