Here is what the output data means:
“version” - Latest version available to download from the server or ‘newest’ if its already up to date.
“stable_version” - Stable version available to download from the server or ‘newest’ if its already up to date.
For API authentication there are 2 methods: Session Token Authentication and HTTP Basic authentication
You can find more information about them here: Fundamentals | Teltonika Networks Web API
If you decide to use HTTP Basic authentication, you will need to enable it under Network → Administration → Access Control, as shown in the picture below