The Dex Nav was a feature in OR/AS only that made it easier to get information about a hidden Pokémon in the Overworld. You can see it's "potential" if it has a special move, and etc. When you beat the post game, you can get more hidden Pokémon! You need to sneak up on a Pokémon that appears via Dex Nav, however.
What did you think of the Dex Nav? Would you like to see it again in the future?