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 HM 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 and used 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.

The Sevii Islands
Islands:	Knot | Boon | Kin | Floe | Chrono | Fortune | Quest | Navel | Birth
Towns:	One Island | Two Island | Three Island | Four Island | Five Island | Six Island | Seven Island
Routes:	Treasure Beach | Kindle Road | Cape Brink | Bond Bridge | Three Isle Port | Three Isle Path
	Resort Gorgeous | Water Labyrinth | Five Isle Meadow | Memorial Pillar | Outcast Island | Green Path
	Water Path | Ruin Valley | Trainer Tower | Canyon Entrance | Sevault Canyon | Tanoby Ruins
Landmarks:	Mt. Ember | Berry Forest | Icefall Cave | Rocket Warehouse | Lost Cave
	Altering Cave | Pattern Bush | Dotted Hole | Tanoby Key | Tanoby Chambers