All tasks have a few options in common: Description, Inactive, and Prevent running if already run recently. Everything else varies from one task type to another. Making a task Inactive prevents it from running, but doesn't stop any task groups it's part of from running other tasks. Setting a task's Prevent running... value will keep any task group that it's a part of from running the task too often. In this example, the task "Generate reminders" won't be run more than every 23 hours regardless of how many task groups it's a part of. Once it's been run by any task group, the next won't try to run it until 23 hours have passed.
You can manually execute a task by right-clicking on it. This bypasses any restrictions and runs it for you even if it's not due to be run anytime soon.