Agreed Kylehnat.
That's a little vague Omnis, but I'll give it a shot.
I think you mean a 'Σ'
It's a summation symbol. You sum the items of the series following it.
Beneath it you'll usually have something like x=i, where i is an integer, x a variable. Above you'll have another integer, j. i is the lower bound for x, and j is the upper bound.
Begin by setting all x's in the equation to the value i, and add to it the result of the equation of x=(i+1), then x=(i+2), x=(i+3), ..., x=j
For example,
for x=2, you get 7
for x=3, you get 13
for x=4, you get 21
for x=5, you get 31
stop there
the answer to the summation is then 72
Forgive me if I'm missing the point here. Maybe you're talking about the use of the summation symbol in infinite series (ie, MacLaurin/Taylor, power, Fourier, etc) - if so, I can try to explain that later for you. The usage in such a situation is the same though. It is used to write an infinite series in a more compact form, usually by setting the upper bound, j, to infinity.