In the derivative operator, you get an expression, and you find its derivative. Now what we want to do is, given some expression, we want to find what it could be the derivative of. If someone were to ask you, what is 2x the derivative of? They're essentially asking you for the antiderivative. You could say, well, 2x is the derivative of x squared. You could also say 2x is the derivative of x squared plus 1. You could also say that 2x is the derivative of x squared plus pi.