Metadata is “data about the data,” such as timestamps, location indicators, author information and much more. If metadata (but not the contents of the data itself) is revealed, it might be more of a privacy issue than you think. Metadata can enable a person put together a suprising amount of information about a person.
Read more here:
http://www.securityweek.com/how-metadata-reveals-more-about-you-you-think