A team's third down efficiency is a measure of how often the team is able to convert a third down into a first down or a score.
The success of a team can be largely dependant on their ability to convert on third down, have sustained drives, and eventually score. The higher the 3rd down percentage the better.
More about 3rd Down Efficiency
There isn't one magic statistic used in determining a team's offensive efficiency, but Third down efficiency is a big one. Consistently converting on 3rd down can also be demoralizing for defenses. Top teams are usually around the 50.0% level for the season.