Focus Sash is a great item if you have a powerful Pokémon that can hit hard but can't hit itself. In the Pokémon Sword and Shield, you can equip it to your Pokémon. It prevents them from dropping after a single hit, allowing them to keep a 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 are 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. Next, gain access to the largest city to the north, Hammerlocke. Otherwise it is blocked until the first three gyms are completed.
When you enter the town, go to the house to the left of the Pokémon Center. It is supposed to be a two story house and when you walk in, there is a man in a blue gi with a red headband on. Talk to him and accept his challenge in 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 of whether you can take him out the first time, he responds extremely harshly. After taking 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 have observed its power during your battle. Make sure you equip a Pokémon with a high amount of attack, which usually doesn't last long in battles. The article disappears after its use, however.
After beating the game, you can purchase it from Wyndon's Battle Tower using Battle Points, which you earn by winning battles in the tower. You may want to hold on to the item until you can reliably have more of it after beating it Sword and Shield.