This article gives you the guideline to understand Event Source in Cloud Alert
What is Event?
Event Structure
How does event match rule criteria?
Rules map triggers to actions (or workflows), apply matching criteria, and map trigger payloads to action inputs.
Cloud Alert uses rules to define workflow as automations. Rules map triggers to actions (or workflows), apply matching criteria, and map trigger payloads to action inputs.
You can refer to Rule engine for more detail.