It's called 24's because each round is a total of 24 reps and a total of 24 sets for the total workout circuit. You can also lighten the weight and just do 25 seconds for each round. But make each movement powerful! 4 sets of 6 reps for each exercise with a barbell recommended, but you can use dumbells as well; no rest in between each exercise.
See workout here https://www.youtube.com/watch?v=3wD6qsUrpEw