A subjective stat. Most college programs define it as a short shot (i.e. a pitch or chip shot) and the percentage of time that they get the ball up and down, or a chip and a putt.
A second up and down is if a player misses the green with their first Up and down opportunity and then has another Up & Down opportunity. The first one would be 0/1 and the second one would be 1/1 for 50% total.
Example: Player misses green, chips it over the green again. They then chip that ball to 5 ft and make the putt.