An event is a sub-set of your participants. Events are typically defined by a combination of a category, course length, age groups and or performance level.
You can have multiple events in your competition, but as a rule of thumb it is a good start to use one event for each group that you want to award prizes too.
If you need to separate certain participants within one event, you can also use tags. This can be useful, for example when a single junior is competing in the elite / senior event.
More about events can be found in the Competition Structure Tutorial.