Each workflow is made up of one or more steps.
When a PR matches a workflow, it will then move through the steps in order, only moving to the next step when it satisfies the requirements for the previous steps.
Visually it gives people a way to see “where the PR is at” and what’s required for it to keep moving forward.
The current step is also shown in the GitHub status check.
The most important aspect of a step is the requirements. The PR must satisfy the requirements of each step in order to move on to the next one.
At least one of your steps will probably include a human review requirement, but you can also include other requirements like passing tests or verifying the title/description/labels of a PR.
Notifications are delivered as GitHub comments, and can be used to communicate with the PR author or reviewers.