Hi there!
Great question and totally understandable. An “event” in Terra is simply one processed payload of new data sent to your backend (like a daily sleep payload, an activity payload or a daily payload, or a user’s authentication status which should only occur once after authentication).
We send webhooks as this guarantees data freshness without you having to poll data manually or limit the amount of data you get - whenever there is a new activity being sent from the data source (e.g. Garmin) to the cloud, Terra sends an update to you! The frequency with which data is being sent to you depends on the provider. We usually observe that the amount of events being received is very stable and consistent across a user, making it very predictable how many events you'll receive each month.
Are you currently testing with your own device or observing these event counts on production? Happy for us to have a look at your usage and help you understand the numbers you are seeing, just ping us a message on our private support channel (right upper corner on the dashboard) - most of our starter projects never run into overage. :)