Question

What is meant if we say “this equation is an identity”?

Answer 1

Identity

An identity is an equation that is true for all values of the variables. For example:

(x+y)^2=x^2+2xy+y^2
The above equation is true for all possible values of x and y, so it is called an identity.

Strictly speaking we should use the "three bar" sign to show it is an identity as shown below. But it is very common to use the equal sign.