The main window shows a draft preview which is useful for editing, but the quality is too low for production. That’s why you need to render the scene when it is done.
To start the rendering click the Render icon in the toolbar:
You will see the settings window, where you can adjust some parameters. Then the rendering itself that may take some time, depending on the settings and the scene complexity. Finally, you’ll see the rendering results and will be able to save the results.