Baiju Solanki has written a great post demystifying Klout scores. Klout scores are those little numbers ranging between 0 and 100, which indicate a person or business's level of online influence. Downloading it as an extension allows you to see it next to any person's public profile.

The takeaways are:

• Klout scores are not the be all and end all, but they're a useful way of working out how engaging and influential you are online. Anything above 40 is good. Anything above 70 is superb.
• Klout scores are mostly drawn from the main four social media accounts: Facebook, Twitter, Linkedin and Google Plus, as well as Wikipedia, Instagram and Four Square. This does not mean that you should be active on all of these channels to do well. As usual, it is better to concentrate on a few channels that you can contribute to meaningfully, than spread yourself too thin.
• Content is key. The best way to improve your Klout score is to create and share valuable content. If your Klout score is currently ranking below 40 or you would like it to improve, I would recommend using a tool such as Passle to create content on a regular basis.