Redberries are items used for creating dyes and making redberry pie. Even though they are food used in cooking, they cannot be eaten unless made into a pie. They are also used in making ranging potions.
Redberry bushes can be found near the south-east Varrock mining site, which have two redberries each. World hopping can be used to gather berries quickly at this location. They can also be grown from a redberry seed at level 10 Farming in any bush patch, which have four redberries each.
You can use redberries to make red dyes. Just ask Aggie the witch in Draynor Village to make red dye and bring her three bunches of redberries and 5 coins as a fee.