What does the semi-circular bar behind a Pokemon mean?

Solution 1:

The bar above a Pokemon represents their CP in a relative range, for that specific species.

The lower end of this bar represents some (still unknown) minimum value for a single species. The higher end of this bar represents the maximum CP this Pokemon can have at your current Trainer level.

For example, Rattatas are rather weak, therefore their CP range is a lot smaller than that of a Drowzee, which explains the bar difference you are seeing.

You may fill the bar up by training your Pokemon. Note that absolutely nothing happens to the Pokemon when you fill the bar -- you've just reached the Pokemon's current CP cap.

Solution 2:

The bar behind the Pokemon represents the Pokemons CP out of the total available CP for that Pokemon.

Increasing the CP level of a pokemon will make it stronger.

From the following page http://gameranx.com/features/id/63505/article/pokemon-go-essential-tips-tricks-answers-to-your-questions/

Once you’ve got a Pokemon, you’ll want to power them up or evolve them, right? Pokemon evolve based on their CP level. CP level is the most important stat for a Pokemon — weight doesn’t seem to affect things at all — and there’s only one way to get more CP. Feeding Pokemon Candy or Stardust is how you’re going to raise their CP, and there are a few ways to get these valuable items:

• Transferring duplicate Pokemon to the Professor
• Rewards at Pokestops
• In-App purchases

CP isn’t raised through fighting in Pokemon GO. This game is all about getting those items — grabbing identical Pokemon to your favorites will provide Poke-specific candies.