In that example, they are dividing the interval [0,1] into 4 equal sub-intervals, like so:
[0,0.25] [0.25,0.5] [0.5,0.75] and [0.75,1]
Next, you need to find the midpoint of each interval, because this is what you will use to plug into f(x) for each sub-interval.
Midpoint of [0,0.25] is 0.125
Midpoint of [0.25,0.5] is 0.375
etc.