In Mechanic, an event represents anything that happens. This could be an order being paid, or a customer record being created, or a fulfillment being delivered.
An event always has a topic, and data (even if the data is null/nil). Event attributes may be referenced in Liquid using the Event object.
Events are fed into Mechanic by the responsible party – for events that are about things in Shopify, for example, the events come to Mechanic from Shopify itself.