From Bulbapedia, the community-driven Pokémon encyclopedia.
The Unknown Dungeon (Japanese: 未知のダンジョン Unknown Dungeon), also known as tag mode, is a wireless feature of Pokémon Mystery Dungeon: Blue Rescue Team that can be activated from the main menu.
When two players have this mode on and are close enough to each other, the first Pokémon of their party shall go into the dungeon. Their Pokémon will have the chance of recruiting Pokémon and finding rare items. Some of these items are otherwise unavailable, such as the Cacnea Spike, Corsola Twig, and Gold Fang. The player can not see the other Pokémon that navigates through the dungeon, but they will get a message when the other player has completed a mission.
The Unknown Dungeon can be repeated an unlimited number of times.