If you create a document on say, the 12 of November 2010, and later open that document and revise, add to it, or make changes to it in any way that is different from the original; then your editing program will record the date that you made the last modification. IE. date modified.
So, date modified doesn’t change when copying or moving. What’s more it’s synchronized with camera clock, so it could be used to sync media files (DSLR video files for example).