This may be the wrong place to post this, but I'm going to ask here anyway...

For ages I've seen people using the sum ($\displaystyle \sum$) symbol with like n-1 on top and i=2 beneath it (or something along those lines), and I understand this is a geometric series, and I feel I almost understand it, but I thought if someone could try and explain it I wouldn't get confused ^_^

I'm probably going to ask my maths teacher anyway, but as it's a Sunday I thought I'd ask on here...

Any help much appreciated I missed the lesson on this in school so I don't fully understand

Oh and move this to a more relevant place if this is the wrong place to post