When you look at the summation notation:

i should enter your mind as 'index' or the indexing number /variable

n is the count, or the the final value of i

so if i=1 and n=5 there will be a math sentence that will be the sum of 5 separate values.

each value is the result of inputting the values of i , and some cases n, into the summation

If you think of stuff in terms of for loops (ah, programming), it's very similar

sum = 0;
n=5;
for i = 1:n
sum = sum + some formula (dependent on n, x, y, z, or the current value of i)
end

Sorry...it's matlab notation, but very similar to C. The colon is basically a integer increment.

__________________
1982 240D, sold 9/17/2008
1987 300D TurboW124.133 - 603.960, 722.317 - Smoke Silver Metallic / Medium Red (702/177), acquired 8/15/2009
262,715 and counting