f of x, if you give me a value, is going to tell you the value of the function at that point. f prime of x is going to tell you the slope at that point. Let me draw that. Because this is a key realization, and it's kind of maybe initially unintuitive to think of a function that gives us the slope at any point of another function. So it looks like this. Let me draw it a little neater than that. That's still not that neat.