Its meant to be about dealing with grief. That's the overall undertone of the film, with the "grief" taking the form of the Babadook and her learning to accept the "babadook" into her life is symbolic of her allowing herself to grieve the loss of her hubby.
It is very much a B material movie though i must admit.
The film I'VE never understood, and i'm sure i'll piss a lot of people off with this, is 2001: A Space Odyssey. I've tried people, i've REALLY tried to understand it's symbolism , it's tone, it's 'message', and the best I can come up with is AI bad, hoomans create AI which turn bad, but Hoomans created Ai so we are our own doom merchants, whoops irony.