The tutor explains the concept of “closed” in math.

If a set is closed under a certain operation, the result you get is still in the set. For instance, the even numbers are closed under addition: when you add two even numbers, you get another one.

The odd numbers are not closed under addition: when you add two odd numbers, you get an even one.

The even numbers are closed under multiplication, but so are the odds.

Source:

Gallian, Joseph A. Contemporary Abstract Algebra, 3rd Ed. Toronto: D C Heath and
   Company, 1994.

Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.