The terminology of Master comes from GitHub and it refers to future development (in this case Odoo v9 and possibly Saas-6).

Online is Odoo's way of saying that if you are an Odoo SaaS customer, you can install and use the module.  Remember that only approved applications can be installed on odoo's SaaS servers.

The source code for the module in question is located in GitHub under the Master branch.  You would need to review the code to see what would prevent it from working in V8 or maybe contact the developer directly.