What are non-cumulative tasks?
Marking a repeating task as non-cumulative will prevent a build-up from occurring on your to-do list. As soon as there is more than one that is "due," the extras will be deleted.
For example, imagine you have created a daily repeating task of "Check the mail"
Then, you go out of town for a week, missing 7 of these repeating tasks.
When you return on Monday, it doesn't make sense that you would check the mail 7 times -- and it would be annoying if when you checked off the 7-day overdue task of "Check the mail," it immediately showed you a 6-day overdue task of "Check the mail." Checking the mail once is sufficient to bring you back up to date with this task: It is non-cumulative.
Is there ever a situation where you would WANT a repeating task to be cumulative? Sure! Let's imagine you set a goal of making 100 paper cranes for a friend, and plan to make 10 a day for 10 days. You create a daily repeating task of "Make 10 paper cranes." If you miss a day, you WILL have to complete this task twice to stay on track with your goal -- otherwise you'd only have 90 cranes on the 10th day! This task is cumulative.