Onboarding is the process of registering an application that has been installed on the Sandbox desktop with the Frame platform. When you onboard an app, you create a record in the Frame back-end that enables the app to show up in the Frame Dashboard and Launchpad. This record is where all of the app-specific settings are stored - which govern how the app will behave when accessed via a browser. After you onboard an application, it is still ONLY installed on the Sandbox - and can only be run in the Sandbox (in the case of Frame Personal -- this is your "desktop"). In order to move your newly onboarded app to production for multiple user access, you must first publish it.
Publishing your app involves creating a complete image of your Sandbox (Step 1 of Publishing), and then copying that image to your production instances (Step 2 of Publishing). Once you have published, all apps that were onboarded from your Sandbox will now be available from production instances. Note that even if you don't add or remove apps to your Sandbox, you can still publish changes made to your Sandbox image - to propagate those changes to your production instances.
Note that the process of publishing will always create new instances and power them on (and thus will incur a one-hour increment of usage for each instance as set by your fixed or maximum capacity setting).