The Focus Sash is a great item if you have a powerful Pokémon who can hit hard but cannot take a hit themselves. In Pokémon Sword and Shield, you can equip it to your Pokémon. It prevents them from going down after a single-hit, allowing them to keep one point in their health pool even if they should have fallen. However, it only works if they have all of their health points at the time they get hit, so you want to give it to a Pokémon with low defense.
To add this item to your collection of useful tools, you need to beat the third Gym leader. After this, you gain access to the larger city to the north, Hammerlocke. It’s blocked off otherwise until you complete the first three Gyms.
When you enter the town, go to the home to the left of the Pokémon Center. It should be a two-story house, and when you go in, there’s a man in a blue gi with a red headband on his head. Talk to him, and accept his challenge to battle
It should be a pretty quick fight. He has a level two Cottonee, which you could probably take out with a single hit. Regardless if you could take it out on the first hit, it hits back extremely hard. After you take out the Cottonee in the fight, Mr. Focus will want to talk to you again.
For your hard work, he gives you the Focus Sash, which you observed its power during your battle. Make sure to equip to a Pokémon with a high amount of attack, who usually doesn’t long in battles. The item disappears after its use, though.
After you beat the game, you can purchase it from the Battle Tower in Wyndon using Battle Points, which you earn by winning battles at the tower. You might want to hold on to the item until you can reliably get more of them after beating Sword and Shield.