Leptin is one of the hormones of metabolism, it works synergistically with ghrehlin, insulin and glucagon. Its main role is to signal satiety when we eat. Studies seem to show that with high carb diets people become leptin resistant so they don't feel the satiety signal they should feel.
I often remind people to be aware of this when they start a low carb diet -- that they may feel hungry at first but as they become leptin sensitive again their hunger will subside. Higher fat and protein consumption should also help with satiety.