A program is a period of time where laps and mileage are tracked. For example, if you are creating a program for the 2018-2019 school year, set the start date of the program to the beginning of the school year and set the end date after the school year is complete. Just be sure the dates cover all scanning sessions.
You can set up multiple programs to track mileage separately. Just create a new program for each event and the app will ask you what program you want the laps credited on.