As Baby Ruby proceeds, this movie becomes more of a psychological, elevated horror than anything else. It focuses on Jo (Noémie Merlant), a mother going through postpartum depression after having a baby girl, Ruby. Sadly, her husband Spencer (Game of Thrones' Kit Harington) isn't the most attentive, leading to Jo's mind playing tricks on her.
Source:https://www.cbr.com/baby-ruby-ending-explained/