What does it mean when the Opportunity is marked as "Open"?

KB opp open

When an Opportunity is marked as "Open" it means that the signup button is enabled and that users are able to complete the signup process to join the Opportunity.  Each Opportunity will have a different signup process that may, or may not, contain multiple steps to complete.  For any questions about the Opportunity and the signup process, you will want to contact the Manager directly.