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. It is calculated by dividing the total number of 3rd downs converted to first downs or scoring plays, and dividing it by the total number of 3rd downs.
This is an officially tracked statistic, and is used by analysts to determine how good a team is at converting third downs. Converting a third down is important because it is typically a team's last opportunity to hold onto possession of the ball. If they fail, they will most likely have to punt the ball away to the other team, or settle for a field goal. Some versions of the statistic do not count scoring plays started from 3rd down. The latter statistics are an accurate measure of how often a team is able to earn first downs.