Ground Zero is the name given to the site of the Twin Towers of the World Trade Center which were destroyed in New York on September 11 2001. It became known as Ground Zero because this phrase means the point on the Earth's surface closest to where an explosive device has been or will be detonated. The phrase was first used in World War II to refer to the American bombing of Japan. In the case of the World Trade Center, of course, the devastation was actually caused by two planes being deliberately flown into the two towers, but the destruction caused was the same and the media began to use the term 'Ground Zero' almost immediately.
The term may gradually fall out of use, now that about half of the 16 acre site has been given over to a memorial commemorating all those who lost their lives on that day.