For more details on setting up a webhook action in a workflow, see this knowledge article.
When the webhook action is set to the POST method, the webhook will POST the data for the contact going through the workflow. The data will be the exact same data you'd get when pulling that contact using the Contacts API. Note: Webhooks set to the GET method will not send any data for the contact, it will simply hit the specified URL.
Webhooks follow these rules for connecting and retrying requests:
Retry-Afterheader if present.
Note: The webhooks used in workflows are completely separate from the webhooks sent for an OAuth app that would be managed through the Webhooks API. For more details about that type of webhook, please see the documentation here.