Can someone explain this movie?

For me the child was an alien.
But the film itself represents the behavior of the bird that appeared at the beginning of the film. It was an impostor egg. The mother hatched it and after the impostor baby bird was born she got rid of the real baby birds, pushing them out of the nest. The mother was forced to raise a bird that was not hers.
The same thing happened with Genna and Tom. Every time the boy was hungry, he would scream like the baby bird.
And just as Tom buried the baby birds at the beginning of the film, he was also buried, leaving the kid only with the mother, just like the baby bird did

Aside from @agaete2305 great explanation, I think the "lore" of the movie itself was regarding an alien species that was luring couples to parent alien children so they appear more human and be able to fool other couples as real estate agents.
The weird place might've been some other dimension.
To what point? I dunno. Maybe to continue their life-cycle.

It was a complicated game.

Well since the film was categorized as a Sci-Fi, I took it was that it's about aliens that integrated in our society unbeknownst to us as real estate agents. And how easy it is for them to capture humans without the old fashion way we have seen of being beamed up into their space ship. So these aliens created a perfect community... of carbon copy houses. Then of course the rest was the aliens conducting behavior experience of humans. In effect we were watching the human couple as lab rats in a alien experiment.

I thought of it as a reverse farm for humans. Raise the 'insect' things young, then discard them without any regard. Like how some insects prey on other insects, pretending to be them, sting them, infest them with their eggs, which hatch and, in this case, devour their ability to communicate as a human, to then continue the cycle.

The entire place was a farm for some insect type alien/interdimensional being etc, which I got from the way it scampered away when struck. But separated by dimensions so none of them could collude, not that it'd have mattered. each kept in a different octagon.

Why they didn't kill the kid eludes me, he was obviously a madness. even at the end, she tried, and hurt it. could have done that a lot sooner. I would have, being there.

not what I had wanted to watch wasting my bottle of vodka though.. Oo