It's saying f prime is representing the derivative. It's telling us the slope of the tangent line for a given point. So if you input an x into this function, into f, you're getting the corresponding y value. If you input an x into f prime, you're getting the slope of the tangent line at that point. Now another notation that you'll see less likely in a calculus class, but you might see in a physics class, is the notation y with a dot over it. So you could write this as y with a dot over it, which also denotes the derivative. You might also see y prime.