The film is essentially a religious allegory. In the early stages, it seems more like a thriller with a wife who greatly values privacy in her husband's remote country home suddenly having to deal with encroachments from two houseguests with no boundaries. But by the midpoint it becomes clear that Pfeiffer and Ed Harris are meant to represent Adam and Eve, their two sons are meant to represent Cain and Abel, Bardem is God and JLaw is Mother Nature or Mother Earth or what have you. The idyllic home is Earth and its very existence is threatened by the arrival of man.
So you have two major sequences where an influx of humans start to fuck with the house. In the first one, a bunch of friends and family members of Pfeiffer's and Harris' arrive for an impromptu funeral. Lawrence is horrified as they disobey her rules, explore the house with reckless abandon, and even cause damage to the foundation of the house. This damage comes in the way of two idiots sitting on a sink counter that she continually warned them was not braced. Water starts to flood the room, Lawrence starts screaming and the people flee. So it is like a Great Flood-esque reboot of the Noah persuasion.
But then Bardem, who is a poet that had writer's block, starts writing again when Lawrence's character becomes pregnant. The resuming of his creative prowess leads to another influx of humans who basically are obsessed with his work and treat him with outright reverence and worship. Lawrence's character is almost due at this point when the masses arrive to see Bardem as their home. This is the sequence people are talking about. As she starts to see her home overrun by people, she wanders from room to room where she finds increasingly chaotic behavior. In one room, people are having a rave. In another, they are looting. On the staircase, they are lined up waiting to be anointed by a follower of the poet. In yet another, people are being sacrificed (execution style) by Kristen Wiig no less who as Bardem's publicist is presumably meant to be some sort of emissary.
Swat cops arrive and try to maintain order but bullets are flying and bodies are dropping. Lawrence is on the verge of giving birth and Bardem takes her to a room upstairs away from the madness. She gives birth to a son and Bardem says he wants to hold the baby. Lawrence refuses, but he sits across from her stoically, clearly waiting for a moment where he can take the child. She falls asleep and he does just that, handing the infant over to the crowd which sacrifices him. Lawrence is absolutely horrified that they did this and that Bardem allowed it to happen. Bardem tries to tell her though that the important thing is that they forgive the people because they are penitent. He points out that they are hanging their heads and crying. She refuses and grabs a shard of glass and starts shanking and slicing people like she's in a GOT episode. One of them gets the drop on her though and knocks her off her feet and she gets pummeled by the crowd while having expletives hurled at her.
Bardem stops the vicious beating, and she then goes to the boiler room in the basement, lets oil spill all over the floor and lights the house up, causing another apocalyptic event that leads to the destruction of the people in the home but prompts Bardem to start the same cycle of creation all over again.