It's just because your battery isn't holding as much charge as it should anymore. If it's still meeting your needs, you can ignore it. If you need more battery time than it's giving, get a new battery. If you get a new one, get it from Apple. Third party batteries aren't a very good idea.
Service Battery: The battery isnt functioning normally, and you may or may not notice a change in its behavior or the amount of charge it holds. Take your computer in for service. You can continue to use your battery before its checked without harming your computer.