How to Find Approximate Area Using Sigma Notation Video ? 5min.com
Areas, Riemann Sums, and Definite Integrals Video ? 5min.com
(video's 21 to 23!)
(and part two!).
I think if you watch these videos in order you'll ace Riemann sums & understand everything perfectly.
By the way, the sum you've shown is not a Riemann sum but it is a summation formula.
There is a way to derive it but I suggest to you to first concentrate on understanding a Riemann sum & what it is then come back to tis forum to ask how to derive this formula.