If you want to sum values across few series, there are two main ways to achieve this:
1. Formula language
You can sum multiple series by typing a simple expression with the '+' sign.
It will add up only common part for all series.
2. Cross Section analysis
Use the Cross section analysis to calculate a sum across series. This is a good option if you want to summarize a lot of series since you can easily include several series at once.
Cross section Sum option works the same as adding series using '+' sign. However, it is worth noting that the result will differ if you untick 'Include only observation where there are values for all series' option. Then Cross section will add values at all available points in time.
In case you want to sum values of one series Sum(series) formula will be helpful. It only sums the values of one series, the output would be a repeated value for all observations, because it returns the sum of all the numbers in the series.
For more about this check: