Find the downloaded dmg file and open it. macOS mounts the disk image.
The disk image window shows the Membrane Server application on the left, and a link to your computer's Applications folder on the right. Drag the application icon to the Applications folder.
Open the Applications folder and double-click the application icon to launch.
macOS may show a dialog asking if you are sure you want to open the application. Click "Open" to confirm.
The server application runs in a command terminal, remaining active for as long as its window stays open.