"I formation" is the name given to one of the most common offensive formations in American football, it was popularized in the 1970s by Nebraska coach Tom Osborne before widespread adoption by NFL teams later on.
In this formation the fullback lines up three yards behind the quarterback, and the halfback (or tailback) lines up approximately two yards behind the fullback--all in a straight line, forming an "I".
This is a very common offensive formation (especially in college football) due to its possibilities as a passing or running formation. It employs a quarterback, halfback, fullback, tight end, and two wide receivers. Due to this grouping, the defense must prepare for both the pass and the run, keeping defenses off balance.
In the I-formation the tailback starts in the upright position, which allows for easier surveying of the defense before receiving the hand-off.