The Permit is available in Pokémon Black 2 and White 2. It is obtained by seeing all of the Pokémon in the Unova Pokédex (excluding event-exclusive pokémon). It allows you into the Nature Preserve.
What is the Permit?
The Permit is an item that you can use in the Mistralton City airport to fly to the Nature Preserve. The Nature Preserve is far away in an unknown location and does not appear on the Unova map. The Nature Preserve contains rare pokémon that can't be found elsewhere. You can also catch a shiny Haxorus there.
How to Get the Permit
To get the Permit, you have to see (but are not required to catch) pokémon number 001 through 297 in the Unova Pokédex. Those are all of the pokémon in the Unova Pokédex excluding the following event-exclusive pokémon: Genesect, Keldeo, Meloetta, and Victini. You do not have to see any of the Generation I through IV pokémon to get the oval charm (such as Bulbasaur, Turtwig, etc.)
After you have seen all of the Unova Pokémon, talk to Professor Juniper in Nuvema. She will give you the Permit.