Firstly, an Event is either a webinar campaign (either one-off or ongoing), an online summit or a video course. On the Basic plan, you can only have one of these running at any given time.
So for example, if you are running a weekly webinar for your business through HeySummit that would count as one active event. If you wanted to host an online summit in addition to your Webinar, you would either need to turn off your webinar or upgrade to the Professional plan where you can have up to 3 Events running at any given time. The Business plan lets you run up to 10 at any given time.
It's also worth noting, that an event is deemed active when it is open for registrations. So in the case of a Summit, it is active whilst the summit is running. Once the summit has finished, it moves from an active to a non-active status. The replays will remain open but registrations will be closed for obvious reasons. An Event in the non-active status does not count towards your active Event count.