Ranarr weed is a herb that can be cleaned at level 25 Herblore. It is the fifth lowest level herb available, yet is one of the most valuable herbs in the game, as it's widely used in the Herblore skill to make prayer potions when combined with snape grass. It can also be combined with white berries to make defence potions, although this is considered inefficient, since prayer potions are much more valuable. To create a potion, you need to add the cleaned herb to a vial of water to create a ranarr potion (unf), and then add the secondary ingredient.
This herb can be grown by players with level 32 in Farming by planting ranarr seeds in a herb patch, which grants 30.5 farming experience per harvest. Because of its low Farming requirement compared to its value, ranarr seeds are often in high demand as a result. It is also an uncommon drop from various monsters.
According to the book Gielinor's flora - herbs, the name comes from the word "rana", meaning "eye-catching" in an ancient Kharidian language, due to the floral display of the plant.
For an exhaustive list of all known sources for this item, see here