A series is a part of a competition. It can be seen as a separated competition within one sporting event. Most events will just have one series, but as RaceGorilla offers an advanced competition management platform, there is the possibility to enter multiple series.
If you just have one series, you can just continue to add categories to your competition.
More about series can be found in the Competition Structure Tutorial.
Note: As series are separate parts of the competition, all timers have to choose the correct series. Participants started in one series are only visible at the split and finish location of that particular series.