When you take the derivative of a function, you are seeking a variation of that function that provides you with the slope of the tangent (instantaneous slope) at any value of (x). For example, the derivative of the function f(x)=x^2 is f'(x)=2x. Notice that the derivative is denoted by the apostrophe inside the f and (x). Also note that at x=0, f'(x)=0, which means that at x=0 the slope of the tangent is zero, which is correct for the function y=x^2.