An academic year and its terms are used to represent your school year and any terms (semesters, trimesters, quarters etc.) that you may have. Both years and terms have start end dates and are used to constrain a class to repeat between those start and end dates.
Depending on your school system, your classes may be consistent throughout the school year or they may change on a term-by-term basis. If, for example, your school system uses semesters, you should create an academic year with two terms.
Academic years also control how your classes repeat. When creating an academic year, you will be asked if your schedule is fixed or rotates on a week or day basis.
Also known as vacations or days off, holidays allow you to specify dates on which you do not have classes. If you have a rotation schedule, you can also use holidays to push rotation weeks or days.
For example, if you have a 2 week rotation timetable and a holiday that lasts a week. The week before the holiday is week 1. When checked the “pushes schedule” would mean the week after the holiday would be the alternate week (week 2) whereas when unchecked it would be the same week as before the holiday (week 1).
If your classes are still appearing in your calendar during holidays, check out this FAQ.