This statistic is used to calculate a kicker's accuracy. An extra point is never automatic, but a starting kicker should be able to make at least 98% of their extra points. Ideally, a kicker will hit 100% of their extra points, since they are taken from such a short distance. But sometimes a missed block assignment, a bad hold, or just a bad day can result in a missed extra point.
The extra point percentage is determined by dividing the total number of extra points attempted by the number of extra points made. The resulting number is converted to a percentage, and you have your statistic. Some kickers have been known to miss an extra point every once in a while, but any kicker who misses multiple extra points in more than 2 games during the year should fear for their job. Many starting kickers in the NFL have maintained a perfect extra point record through multiple seasons.