Let A be the set of prime numbers less than 2. What are the elements in A? No prime number is less than 2. So no prime numbers can belong to A. This means there is no element in A.
Let B be the set of all perfect squares which are negative, we know that perfect squares are all non-negative. So no perfect square belong to B. This means there is no element in B. Such sets are called Empty set.
A set containing no element is called a null set or a void set. It is denoted by ∅.
Empty set is a subset of every set. This means ∅ ⊆ A for every set A.