What it means is, check the picture below.
so, the first part shows two rectangles... the rectangle goes from 1.0 to 1.6, so it has a width of 0.6.
and the height comes from the middle point, or 1.3 is f(1.3) = (1.3)² or 1.69.
so you get the area of the first rectangle, 0.6 * 1.69
and then you do the same for the second rectangle, it goes from 1.6 to 2.0, so is of width 0.4, and its height comes from the middle 1.8 point, is f(1.8) = (1.8)², or 3.24.
so its area is 0.6 * 3.24
then you add the areas of the rectangles to approximate the area under the parabola.
so, you'd do the same for the other part of the picture with the four rectangles.
the first rectangle of the four for example, has a width of 0.2 and a height of f(1.1) = (1.1)² or 1.21.
so, the first part shows two rectangles... the rectangle goes from 1.0 to 1.6, so it has a width of 0.6.
and the height comes from the middle point, or 1.3 is f(1.3) = (1.3)² or 1.69.
so you get the area of the first rectangle, 0.6 * 1.69
and then you do the same for the second rectangle, it goes from 1.6 to 2.0, so is of width 0.4, and its height comes from the middle 1.8 point, is f(1.8) = (1.8)², or 3.24.
so its area is 0.6 * 3.24
then you add the areas of the rectangles to approximate the area under the parabola.
so, you'd do the same for the other part of the picture with the four rectangles.
the first rectangle of the four for example, has a width of 0.2 and a height of f(1.1) = (1.1)² or 1.21.