I think it makes the Pokemon that hold it become happy more easily but im not sure
I also heard that it is useful to chansey as it can help chansey evolve to blissey.
That's not all, the most cool is that if your Pokemon have it equipped, it gains hp when it hit opponent.
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This item can be found in Mt Pyre.
If you put Soothe Bell on a Pokemon or give it Pokeblocks.
Makes your pokemon's happiness go up faster.
In salport city there is a woman looking at a picture in the Pokemon fanclub. Make sure the first Pokemon in your party has full happiness's. Then talk to her and she will give you the soothe bell.
You find a soothe bell at the Pokemon mansion