Razz Berry

In-Game Description
Feed this to a Pokémon, and it will be easier to catch on your next throw.

Details
Razz Berry is an item that can be found at trainer level 8 and higher. They can be obtained from PokéStops or by reaching certain levels.

Feed this berry to a Pokémon while trying to capture it to increase the chance of success.

The in-game description of the item is misleading where says "on your next throw." The effect actually lasts until the Pokémon "enters" the ball at least once. If the Pokémon seems to be captured, but escapes the ball, the effect of the berry wears off, and another must be used. If the Pokémon does not enter the ball, such as if your throw misses, the effect does not disappear.