From Bulbapedia, the community-driven Pokémon encyclopedia.
The Key System is a feature of the Unova Link introduced in Black 2 and White 2. It allows players to change the difficulty level of the game, switch between Pokémon Black and White' version-exclusive cities, and change the Mystery Door in the Underground Ruins.
For the first time in a Pokémon game, there are different difficulty levels. Only the "normal mode" is unlocked by default; the "easy mode" and "challenge mode" must be unlocked through the course of the game.
The easy mode decreases the player's opponents' artificial intelligence and the level of their Pokémon. As such, battles become easier. This mode is unlocked after beating the Champion in Pokémon White 2.
Challenge mode increases some opponents' artificial intelligence and the level of their Pokémon. Some opponents will also have an increased number of Pokémon. This mode is unlocked after beating the Champion in Pokémon Black 2.
To unlock easy mode in Black 2 and challenge mode in White 2, the respective keys for each version needs to be transferred from another player via the Unova Link.
The version exclusive cities, Black City and White Forest, are found by default in Black Version 2 and White Version 2, respectively. They can be unlocked in the opposite game if certain conditions are met.
After completing five areas of Black Tower in Black 2, the player will be rewarded with the Tower key. This can be transferred to White 2, which will unlock access to Black City. Likewise, after White 2 players have completed five areas of White Treehollow, the Treehollow key can be transferred to Black 2. This unlocks White Forest for Black 2 players.
Players of either version can use the Unova Link to switch between cities once both are unlocked.
A mysterious door in Clay Tunnel gives access to the Underground Ruins, where the legendary golems can be found. Only one golem will appear at once; Regirock is the default Pokémon in both Black 2 and White 2. Catching this Pokémon will see Black 2 players rewarded with the Iron Key and White 2 players with the Iceberg Key; these allow access to Registeel and Regice, respectively.
While Registeel and Regice are native to Black 2 and White 2 respectively, players of the opposite version games can obtain their final legendary golem by transferring the appropriate key through the Unova Link.