"There are specific actions that lead to your customer getting results from your product. These specific behaviors are simple and easy to recognize. When your customers take these actions, they will achieve increased results. If they don't take these actions, you know the customer is at risk."
Example
For a SaaS product, Action Milestones might include: the customer integrates your product with their tech stack; the customer creates their first artifact in the product; the customer shares something they built with their team.