Terastal, or Terastallization, is the Generation IX battle transformation in which a Pokémon crystallizes once per battle and changes its defensive type to its single Tera type, gaining STAB on that type while keeping STAB on its original types. There are 18 standard Tera types, plus the Stellar type as a 19th.
In Pokémon Scarlet and Violet any Pokémon can Terastallize once per battle after you earn the Tera Orb, and the Orb recharges at a Pokémon Center or at Tera crystals. Defensively the change is total: a Dragonite that Terastallizes into Steel is no longer weak to Ice, Dragon, or Fairy, because while Terastallized it defends as a pure Steel type. Offensively it keeps STAB on its original Flying and Dragon moves and gains STAB on Steel moves, or a stronger 2 times bonus when the Tera type matches an original type.
Every Pokémon has one fixed Tera type, shown on its summary, and wild Tera Pokémon found glowing in Paldea can have unusual types for their species. You can change a Pokémon's Tera type at the Treasure Eatery in Medali using 50 Tera Shards of the type you want. Tera Blast is the signature move of the mechanic, changing type to match the user's Tera type when Terastallized.
The Indigo Disk expansion added Stellar as the 19th Tera type. Stellar gives no defensive change, keeping your original weaknesses, and instead boosts the first use of each move type before that boost is spent, with the limit lifted in Tera Raid Battles. It is a one time spread of power rather than a new defensive plan.
In play"Terastal into Fairy here and the Dragon move that should kill you does nothing."