This right here is delta x, so that is a delta x. This is another delta x, this is another delta x. A reasonable way to conceptualize what dx is, or what a differential is, is what delta x approaches if it becomes infinitely small. So you can view this, you can conceptualize this, and it's not a very rigorous way of thinking about it, it's an infinitely small delta x. It's one way that you can conceptualize this. So once again, you have your function times a little small change in delta x, and you are summing, although you're summing an infinite number of these things from a to b. So I'm going to leave you there, just so you see the connection, you know the name for these things, and once again, this one over here, this isn't the only Riemann sum, in fact this is often called the left Riemann sum, if you're using it with rectangles, you can do a right Riemann sum, you could use the midpoint, you could use a trapezoid, but if you take the limit of any of those Riemann sums, as n approaches infinity, then that you get as a Riemann definition of the integral.