it doubles the happiness a pokemon gets. so instead of 20,000 steps it would take you 10,000 steps.
hope i helped...
The sooth bell makes the pokemon want to rap.... anyhow, I am not sure of exact number, but on platinum I walked about 20k of steps with a sooth bell equipped and it made it full happiness.
Give it to a Pokemon and they will become friendlier over time. It increases it by some but the most effective way to increase happiness is to feed your Pokemon vitamins such as Protein, Iron, Carbos, etc. I believe you should only feed a maximum of 10 vitamins before your the vitamins have done as much as possible for your Pokemon's happiness level.
The eyes are AMAZING! I always coapmlin that my kids eye's never look great because they are brown but now I know I just am not shooting and editing right!