Athelas is also known as Kingsfoil or Asëa Aranion.
It was first brought to Middle-earth by Númenoreans, but by the end of the Third Age the knowledge of its healing properties was lost among all but the Rangers of the North. According to folklore in Gondor, it is especially powerful in the hands of the king, perhaps because of the Elvish heritage of the royal house. It was used by Aragorn, secretly entering Minas Tirith upon his return to Gondor to heal those touched by the Black Breath, an act that enhanced his reputation and strengthed his claim to the crown.
Athelas was also used by Huan and Lúthien to heal wounded Beren in the early Lay of Leithian. It contradicts the information from The Lord of the Rings of it being brought to Middle-earth by Númenoreans, so it was either changed by Tolkien in later versions of his legendarium, or athelas grew in Beleriand before it was destroyed, and then brought back to Middle-earth by Númenoreans in the Second Age.
Appearances outside the legendarium