Question

Integers are closed under addition. What are closed property means?

Answer 1

A set has closure under an operation if performance of that operation on members of the set always produces a member of the same set;
in this case we also say that the set is closed under the operation.

For example, the positive integers are closed under addition,
2+3 =5
Here both 2&3 are positive integers and on adding we get 5, which is also a positive integer

but not closed under subtraction:
1 - 3 = - 2
​​​​​​Here - 2 is not a positive integer even though both 1 and 2 are positive integers.