The PokéNav is a multi-purpose item that was introduced in the third generation of Pokémon video games (Pokémon Sapphire, Ruby and Emerald).
It is donated to the player through the Devon Corporation's President, once he asks you to complete a task, and in favour for the other tasks and favours that the player has done for Devon Corporation.
The PokéNav is accessed not through the player's Bag's Key Items pocket, or any other pocket in the Bag, but it's an option in the Start Menu. PokéNav stands for Pokémon Navigator. It has a map of the Hoenn region, with both a full map, and a close up map that highlights more specific places than just Towns and Cities. It also has a 'Condition' section, which allows the player to check party Pokémon's status, i.e. those which have been enhanced with PokéBlocks. It also has a 'Search' function in the Condition section, for narrowing down results. It contains a 'Trainer's Eyes' function, allowing the player to see all of the Trainers that he / she has battled, with lots of detail on that trainer. It also shows the trainer's battle sprite.
You may make calls to people for a rematch or to just talk, in Pokémon Emerald you may even get Gym leader's phone numbers for a rematch and meet them at the gym, you may also see what things a trainer likes to do and does on a daily basis. Sometimes important calls can be made from other people to let you know of an important event that might be coming up or just to warn you about something. You may call Professor Birch to check on your Pokédex and see how it is and compared to the National Dex as well. This phone/navigator is very simalar to the Pokégear in Heart Gold and Soul Silver as you can call people for a rematch and use a map whenever you want to. You can use the PokéNav map for things like tracking down Latias and Latios or to simply tell where you've been.