From Bulbapedia, the community-driven Pokémon encyclopedia.
A monster house is an event that sometimes occurs in some dungeons of Pokémon Mystery Dungeon. They are found in certain rooms of dungeons usually containing many items. Once the player enters the room or path, many Pokémon in large numbers will drop down from the roof and start to gang up and attack the player. At this point, the music will change into a more menacing tone. The Pokémon inside are always ones that can be found in the dungeon it is located in. A monster house will also increase chances of a rare Pokémon to appear. Monster houses only appear in later dungeons.
The player will realize that they are in a monster house when the music changes and Pokémon sudden pop up all together. If the player tries to escape the Pokémon of the monster house will follow until they are all defeated. Another way that a player may realize that they are about to enter a monster house is when their partner Pokémon stops following them. One of the skills of all IQ groups but C, D, and G have the ability House Avoider, causing them to avoid monster houses.