RAZZBERRIES razzberries - thepokemonplace.com

Razzberries  fruit you can use to try to make hard-to-catch Pokémon easier to catch. You simply Throw one out during a capture scenario and hopefully it helps contain them inside of the PokeBall. Razzberries help gain the trust of the Pokemon and once you’ve gained their trust, it should make catching them alot easier. RazzBerries dont start appearing until later levels. this is  when more difficult pokemon start showing up.
Instructions on using a RazzBerry:

If a pokemon has a high Combat Power(CP) , or is rare, the circle for throwing will  be darker in color, it will range from orange to red, this indicates the pokemon may escape when first caught or even worse, run away. You can instead use a RazzBerry first to toss at the Pokemon to get it to like you or trust  you more and therefore increase the chance it will stay in the next Ball thrown at them. You Can not purchase Razzberries, but have no fear, they will start appearing in PokeStops and level Rewards after level 8.