Scheduled maintenance refers to manufacturer's recommended time or mileage intervals to change certain hard parts of your vehicle. Some of these are shown below. Generally speaking replacement of these items can be scheduled in advance for both time and funds budgeting. Some items are more critical than others. As an example, timing belt replacement is an expensive maintenance task but failure to replace the timing belt as per manufacturer's recommendation can lead to catastrophic results with engine damage and extensive repair bills should the belt break or get stripped. There is a video below on why it is essential to change a timing belt on a timely basis.
Here is a video on why timely replacement of the timing belt is essential!