The idea is that you enter you birth date, country, and sex, and the application will provide you with a number - for example, 4,534,132,121 - which is an estimate of how many people were alive on the date of your birth, along with other statistics. Many people are playing around with this cool application, posting their "number" on Facebook or Twitter.
While this application is fun to play with, BBC has done a brilliant thing here: People from all over the world - tens or hundreds of thousands, perhaps even millions - are voluntarily sharing private information with the website. The date-of-birth and sex, along with the user's IP address, is valuable information. And people are unlikely to fake the information here - they are dying to know their "number". So this information is most likely very accurate, and thus very valuable. The BBC is amassing a small treasure here. There a good reason to believe this information will be used by the BBC for their own commercial purpose, which might be annoying, but probably not harmful. However, if the information is shared with governments, or if there is a security breach and it's stolen - and this is not a rare event - and the data falls into the wrong hands, there's some chance that the information will be abused.
My recommendation: Be mindful. Think twice before sharing your personal information on the web.