The herb Aragorn uses to help heal Frodo after Weathertop is athelas.
Also called kingsfoil, athelas is a healing herb associated with Númenórean kings. Aragorn gathers and prepares it after Frodo is stabbed by the Witch-king's Morgul blade at Weathertop. The herb cannot simply undo the wound's dark magic, but its healing properties help slow the injury's effects while the party races toward Rivendell.
Aragorn's knowledge matters because ordinary people in Middle-earth have largely forgotten how to use athelas. In Bree, the herb is treated as a common or nearly useless plant, while Aragorn recognizes its value. Its fragrance and restorative power become signs of his heritage and healing skill.
Athelas appears again later in the story. Aragorn uses it to treat injuries among the Fellowship, and its scent helps revive Merry after the Battle of the Pelennor Fields. A common mix-up is confusing athelas with lembas, the Elves' waybread, or simbelmynë, the white flowers associated with Rohan's royal tombs.