Do you remember what perfect squares are? When a number is multiplied by itself, the product will be a perfect square

So numbers 0, 1, 4, 9, 16, 25, 36, 49, 64, 81, 100 etc. are called perfect squares Similarly you can multiply an algebraic expression by itself to get a perfect square expression

Square of a binomial

(a + b)² = a² + 2ab + b²
(a − b)² = a² − 2ab + b²

When a binomial is multiplied by itself it gives a trinomial which is a perfect square

NOTE: (a ± b)² ≠ a² ± b²