This statistic is calculated by subtracting a team's starting position on the field from the team's ending field position, and adding any punt or kickoff return yards to the total. It is tracked by the League, individual teams, and sports publications. Many stat sheets list only "Yards" when laying out statistics for a team's total offense; this is often net yards.
The concept driving this statistic is the importance of determining exactly how far a team was able to advance the ball. Penalty yards are automatically incorporated into the equation because any yards added or subtracted would affect the team's overall field position. If a team returns a kickoff from the goal line to their own 20 yard line, and advances the ball to their own 40 yard line, their net yardage for that series would be 40 yards (40-20 + 20 yard kickoff return = 40).