This movie is about coming to grips with the inexplicable. It is an epistemological treatise wrapped in the shell of a drama about grief, loss and uncertainty, with a touch of the supernatural.
The performances are perfect. The main characters have small lives in a small town. They react to events in ways that are relatable and universal. There are no reused cliches from other movies and the film retains its internal logic throughout.
The theme of grief is secondary to something we can all relate to. When someone close to us passes, there are so many questions and conversations we wish we would have had and, as human beings, we have to come to terms with the possibility that there may be no possibility of receiving answers.
The supernatural nature of the film is just a metaphor for possibility. Whether or not you accept the possibility is up to you.
The film also is about how we package events to form patterns. We do this by formulating a hypothesis and then fitting the data to our hypothesis. The final segment stakes a claim on the validity of the characters hypotheses, but intelligent viewers are free to create their own endings.