The Choice Scarf is a held item that boosts a Pokémon's speed stat by 50%, while only permitting the use of one of the Pokémon's moves. It is one of three "Choice" items in the main series games, joined by the Choice Band and Choice Specs.
Choice Scarfs increase the Pokémon's speed stat by 50% in battle and are, in most cases, used by competitive players to turn already fast Pokémon into threats capable of outpacing any of the opponent's team members.
How to Get a Choice Scarf in Pokémon Scarlet and Violet
In Mesagoza, before reaching either the Naranja or Uva academy (version depending), players will find a series of shops scattered throughout the city. Here they can purchase various items that can help in different ways throughout their journey. One of these shops, found by the west entrance to the city, is Delibird Presents, which sells battle items, Poké Balls and other useful items.
Going to the "Battle items" section in Delibird Presents' Mesagoza branch, after completing the game's main story, allows the player to buy the Choice Specs, Choice Band and Choice Scarf items. Each one sits at 100,000 Pokédollars.
For this price, it's clear that this item can be a battle-changer for whichever Pokémon it's given to. Used on the right Pokémon, it might be able to avoid being outpaced entirely.
After purchasing, the players can find the item in their bag and can give it to one of their Pokémon to hold, thus having that Pokémon experience a significant boost to their speed stat while in battle.