Razz Berries are a less common collectible item that can be used to capture Pokemon much easier. When you are fighting against Pokemon, you will find that some of the Pokemon that have a higher CP will be much more difficult to capture. This means that when you throw a ball at the Pokemon, it may break back out of the ball after you capture it. During the battle a colored circle will appear over the pokemon when you are throwing a Poke ball at it. If the circle is green the Pokemon will generally be much easier to capture. When it turns to orange or red, you may need to use a Razz Berry to capture the Pokemon much easier.
Razz berries will start spawning at Pokestops once you have reached level 8. From here you can start using them during the battles to catch Pokemon much easier.