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 birthday 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.