From Bulbapedia, the community-driven Pokémon encyclopedia.
The Tanoby Key is a puzzle in Sevault Canyon on Quest Island in the Sevii Islands. Unlocking it is the key to releasing the Unown.
In the games
In FireRed and LeafGreen Versions, the key is a small puzzle involving the HM04 (Strength) that, when completed, allows Unown to be captured in the Tanoby Chambers. The puzzle consists of several boulders that must be maneuvered into a correct pattern that will unlock the Unown, allowing them to be caught.
In the manga
In the FireRed/LeafGreen saga, Storc managed to unlock the secret of the Tanoby Key and managed to gain control of the Unown, using them to defeat Lorelei in a Pokémon battle. Later on Storc had them guard the Trainer Tower when she and her companions found out that Mewtwo had returned and was teaming up with Red to defeat them.