In the formula of weighted stock price average, there is an inverted M-shaped mathematical symbol with N above and i= 1 below.
The inverted m you said is a summation symbol. I= 1 means to take values from 1 until n, and then sum the results of each value. For example, i= 1 under m and n=5 above, and the function is 10i, which is =10 *1+10 * 2+10 * 3+/kloc-0.