The NFL Combine is an annual evaluation of prospective college football players entering the NFL draft. Scouts and coaches assess athleticism and skills in a variety of position drills and standard physicial tests such as the bench press, 40 yard dash, shuttle drills, vertical leap and broad jump tests. With so many athletically gifted individuals to sift through, speed (40 yard dash) and explosiveness (jumping tests) can often be the determining factor in who gets drafted early and will ultimately earn the big bucks.
Here is Byron Jones destroying the field and the previous broad jump record. The broad jump is a horizontal leap from two feet in a stationary position. The broad jump is such a good indicator of power and speed, as it is a marker for explosive and complete hip extension.
This is simple enough for anyone to try. Give it a shot and you'll begin to understand how impressive 12 feet 3 inches is.