The salt changes the muscle fibers and allows the proteins in the meat to retain moisture within the cells.
Here's the recipe I use:
1/2 cup brown sugar
1/2 cup kosher salt
4 cups hot water
1 small onion, chopped
1 tablespoon whole black peppercorns
2 cloves garlic, coarsely chopped
4 cups ice water
1/2 cup white wine
In a large metal bowl, whisk together the sugar, salt, and hot water, until the sugar and salt are dissolved. Add the onion, peppercorns, garlic, ice water, and wine. Stir well. Add the chicken to the brine.
You can do this for an hour and it's great. If you brine it over night or all day, reduce the salt a little. You will notice how much juicier the chicken is. Amazing.