Stages are a specific point in a customer's life cycle for the purpose of organizing. They are a fluid label that allows to quickly make use of your prospects.
By default, Outreach comes with 8 stages that you can edit (located under Settings > Prospect > Stages). Outreach admins can add more stages to sync with current Lead and/or Contact Statuses in Salesforce, or simply to better fit your workflow.
All stages are found in the main prospect page. Click into them at any time to filter prospects in the stage.