There's a lot of speculation and theories around the net what it's all about. I don't know where they get these absurd theories when it's plain and simple. Warning spoilers ahead. So here we go:
The real start of the movie is the car crash. The bad mom who abuses her
kid died. Both of them died. The mom was taken by Death but she cheated death. She promised to come back to Death but she never did. She thought she could outsmart death. So no matter what she did it always goes to the car crash scene. You can never cheat death. And that's was so scary. In the TV series Preacher, this theory of hell is also shown. It's the same nightmate over and over again. The film also talked about Sisyphus, which was already a clue of what is going on. Sisyphus was a god who cheated death and as a punishment he carried a boulder to a hill over and over again. Which happened to the mom.
So there. The only thing that's off in the film is the mask which is similar to Timecrimes. Still this is one of my favourites this year.
The movie would be 4* or more as a Sci-Fi instead of the many Horror lists it finds itself on.
Edit: I newly watched a Spanish movie called "Timecrimes" which had a lot with this one in common. Timecrimes however was made in 2007 so this one was almost a remake of Timecrimes.
The only difference was that Timecrimes was simpler and linear with basically everything explained. Triangle plot however occurs within a loop with lesser explanation provided which was more thrilling and mysterious and elevated the experience of the 2nd viewing. In direct comparison I would say Triangle is the better movie but Timecrimes has credit for being more unique. I would recommend both movies.