A version is a static copy of a script. Once a version is saved, it can no longer be modified, only deleted. This feature is used when you are working on a script that goes through many changes and iterations. Versions allow you to keep track of your changes. Furthermore, creating versions is required when you are writing a library. For more information, see Libraries.
Creating a version
Open the script that you are working on and follow the steps below to save a version of it:
Select File > Manage Versions...

- Provide a description which serves as a changelog and then click Save New Version to save the current script as a version.
Once the version is saved, it shows up at the top of the list of versions right below the description box:

Deleting a version
- Select File > Manage Versions...
- Click X to the right of a version to permanently delete it.