Clearly write what it means for a set to be closed under an operation

See explanation below.

Step-by-step explanation:

When a set is closed under an operation, it means that, given a set, performing the defined operation on that set, the results of this operation will be members of the set too.

For example, the positive even numbers are closed under the operation of "multiply by 2". This means, that when we take a positive even number and multiply it by 2, the result will be a positive even number too.