Original: In many countries, circulation figures are audited by independent bodies, such as the Audit Bureau of Circulations and BPA Worldwide, to assure advertisers that a given magazine does indeed reach the number of people claimed by the publisher.
Simplified: In many countries, circulation figures are checked by companies that do not work for the magazine, such as the Audit Bureau of Circulations, to make sure that the numbers a publisher gives to advertisers is accurate.