Hi WindWarrior ,
Solutions are ways to distribute standalone applications, or to distribute customizations of existing applications that are in an environment. The Accelerator is a managed solution that customizes the Project Power App that is already installed in an environment.
If you're looking to build your own applications or customizations, take a look at powerapps.com for documentation.
As a way to quickly get started, you can convert the Accelerator into an unmanaged solution that you can customize .Unzip the Accelerator file, open the customizations.xml file, change the <managed>1</managed> property from 1 to 0, save the file, and then zip the folder up again. After you do this, you can import the Accelerator into an environment and edit the customizations. Be sure to export it as a managed solution when you're done before using it in any production environments.