The Strength Belt is a great item if you have a powerful Pokemon that can hit hard but can't hit on its own. In Pokémon Sword and Shield, you can equip it to your Pokémon. This prevents them from falling after a hit, allowing them to keep a point in their health pool even if they should have fallen. However, this only works if they have full health at the time they are hit. So you want to give them to a Pokemon with low Defense.
To add this item to your collection of useful tools, you need to beat the third Gym leader. After that you get to the biggest town in the north, Hammerlocke. Otherwise, it is blocked until you complete the first three Gyms.
When you enter the town, head to the house to the left of the Pokemon Center. It should be a two-story house, and when you walk in there's a man in a blue gi with a red headband on his head. Talk to him and accept his battle challenge
It should be a fairly quick fight. He has a level two Doudouvet, which you could probably take out with a single hit. It didn't matter if you could get away with it on the first hit, it made coming back extremely difficult. After you take out the Doudouvet in the fight, Mr. Focus will want to talk to you again.
For your hard work, he gives you the Strength Belt, which you observed during his battle. Make sure to equip a Pokemon with a high amount of attack, which usually doesn't last long in battles. The item disappears after it is used, however.
Once you've completed the game, you can purchase it from Wyndon's Battle Tower using The Battle Points, which you can earn by winning The Battles at the Tower. You might want to keep the item until you can get more after beating Sword and Shield.
