The sum is usually given by an index, which appears below its symbol (and sometimes at the top as well). This index ranges from the first value (written below the symbol) to the last value (written above).
For example: summation of expression 2i, for i ranging from 1 to 100. We write the summation symbol, below it we write i = 1 and above it we write 100.
To calculate the result, simply replace i with the value corresponding to the index and add: 2 (1) + 2 (2) + 2 (3) + 2 (4)… + 2 (100).