CP simply means Combat Power, and it’s the number above each wild Pokemon. It indicates their battle prowess, and as you gain more experience points (or XP) and level up as a Trainer, the CP of the Pokemon you find will become higher.
We’re still learning about Combat Power, but here’s what we know so far. In an individual Pokemon’s stat screen, you’ll see a semi-circle at the top. The part that’s bold with a dot at the end indicates that Pokemon’s current maximum Combat Point potential. Some Pokemon have higher maximum CP than others. For example, an intimidating, spiky Pinsir will likely be stronger than a tiny, wormy Caterpie. But, if you find a 50 CP Pikachu, that doesn’t mean it’s stronger than a 20 CP one. For efficiency’s sake, though, you’ll probably want to Power Up the 50 CP Pikachu with Candies.
Whether you Power Up or Evolve a Pokemon first is up to you. Some believe powering up a ‘Mon before evolving it results in even higher CP, but there’s no confirmation of that yet. But, your Pokemon’s max CP rises along with your Trainer level, so don’t neglect those curve balls (more on that shortly) and be sure to visit Pokestops whenever you can! Get dat XP!
Extra Note: It looks like your Pokemon’s weight/size (XL or XS) could affect their stats, like CP, HP, and dodge speed. Again, there’s a lot of rumor mongering and unconfirmed info out there at the moment, so that could be a bunch of Bouffalant hockey.