… addition.
A set is closed under addition if you can add any two numbers in the set and still have a number in the set as a result.
… (scalar) multiplication.
A set is closed under (scalar) multiplication if you can multiply any two elements, and the result is still a number in the set.
Generally, “closed under some operation” means the elements of the set are not being able to “escape” the set using that operation. The result always