The Greek letter sigma () is used as a compact way of formulating sums, and is called a summation symbol. The terms that are summed up have indices and form a sequence. Below the summation sign, you have the number which represents the first index, and above it, you have the last index.
If you, for example, want to add the square of all integers between and , you can formulate this as which means that you first let equal then , and so forth, and then add the terms:
The summation sign does not give the value of the calculation but is just a shorter notation for the sum. The actual calculation is done with mental arithmetics or digital tools.