I'm not entirely sure without reading it, but it's probably some of the dragons that were introduced in the first book or new important dragons in the story.
It's likely to continue the adventure and story of the dragons in the 'Wings of Fire' series. Maybe it shows more about their battles, relationships, and the growth of the characters.
One possible theme could be friendship. As the story progresses, the relationships between the characters might grow stronger, showing the importance of having friends in difficult times. Another theme could be survival, especially if the dragons are facing new threats or challenges in this book.