The Summation Symbol

The summation symbol  
\[\sum\]
  means add, so that  
\[\sum x\]
  meams 'add all the  
\[x\]
's.
The summation symbol is often accompanied by indices top and bottom which indicate which  
\[x\]
's to add.

summation symbol

So for example,
\[\sum_{i=3}^7 x_i=x_3+x+4+x_5+x_6+x_7\]

\[\sum_{i=3}^7 i^2=3^2+4^2+5^2+6^2+7^2\]