The player will act as a ghost that resides within the summerhouse that the family lives in. You can follow them around and see what the family is doing. But of course this is not some voyeur game where you creepily watch a family living their lives.
Aside from watching what they're doing you can learn about them, read their thoughts and of course, affect what they're doing. Will you help Dan to finish his work? or maybe help him with his family? The choice is in your hand. You help this family to evolve whether it is for the better or the worse. Anything you do will affect the story.
This is a really interesting concept and I'm really interested to see how the actual game will play. From what I've seen in the trailer it looks intriguing and might offer a good storytelling experience while still having the amount of interactivity a game needs.
The game seems really promising and you should vote yes for The Novelist on its Steam Greenlight page.