Let's say we have a sequence of $y$ values, heights of the graph. For example $0, 1, 4, 9, 16, \ldots$. And let's calculate the differences which we call $s$. We have $(1 - 0), (4 - 1), (9 - 4), (16 - 9), \ldots = 1, 3, 5, 7, \ldots$. This step is basically differentiation. Now we could ask the question what if we have $s$ how could we go back to $y$? Well, $(0 + 1), (1 + 3), (1 + 3 + 5), (1 + 3 + 5 + 7), \ldots = 1, 4, 9, 16, \ldots$ so that gets us a pretty long way. This step is known as integration.