A set is usually defined as a collection of elements. In this case, U={1,2,3,4,5} is a set that contains the elements 1,2,3,4, and 5. A subset is a set that has been formed by elements of another set. Any selection of the elements of the set {1,2,3,4,5} can be a subset of U. For example, {1,2} is a subset of U, because it is formed by elements of U.
In this case, A={2,3} is another set, whose elements lie inside set U. Then. A is a subset of U.
