Pokédex is considered as the encyclopedia of Pokemon, where the description, height, weight, species type and other information of a certain Pokemon is being listed. As a Trainer, you will progress in levels, once you gained a higher levels, you will be able to capture more powerful Pokemon and add it to your pokemon all regions map to complete it. Additionally, you will gain access to much more powerful items like Great Balls to provide you a more effective hit in catching a Pokemon. In order to move up into advance level, you'll need to explore and discover more Pokemon.
As the game progress, you'll discover alternative strategies to get more Pokemon to your Pokédex. In the event that you capture a Pokemon at a sufficient intervals, you'll get the chance to develop and make one of them to a more stronger Pokemon. If you keep exploring, you would probably come across Pokemon Eggs found at PokeStops, that will hatch when you're already at a particular range. After they hatched, you might possibly discover Pokemon which you may perhaps have never encounter previously.