Backfield

The backfield is the area of an American football field behind the line of scrimmage. The backfield or offensive backfield can also refer to members of offense who begin plays behind the line, typically including any backs on the field, such as the quarterback, running back, and/or fullback.

Play in the backfield
There are many rules which differ for play in the backfield as opposed to play in front of the line of scrimmage. A forward pass can only be legally made from the backfield. Most running plays begin with a handoff in the backfield.

If the offensive ball-carrier is tackled in the backfield, the team will lose yards, in that the distance they need to attain for a first down is more than at the beginning of the play.