iirc they implemented it during the same time as the B2/W2 update?
Basically, if you tucked in a Pokemon with certain Type, the chance of visiting a specific area greatly increases.
Here's the list of the areas and the types:
Pleasant Forest : Grass, Electric, Poison, Bug
Windswept Sky: Flying
Sparkling Sea : Water
Spooky Manor : Psychic, Ghost, Dark
Rugged Mountain : Fire, Rock, Ground, Fighting, Steel
Icy Cave : Ice, Steel, Ground
Dream Park : Normal
No preference : Dragon
If a Pokemon has dual typing, both types are applied, but it seems that primary type has priority, so a Grass/Flying will give you a higher chance to visit the Forest, slightly higher chance to visit the Sky, and lower chance to visit everything else.