Game versions is where you configure your game’s versions and the API versions that match them respectively.
You can choose to have multiple versions, or just one that you update each time - this all depends on how your game actually works.
For games like Minecraft, for example, there are multiple concurrently active versions. Other games only have one active version, that updates each time you release new content for your game.
Each game version should have a corresponding API version, so that mod authors, who are creating content for your game, know which API version to query when attempting to communicate with your game
If you use a single game version, you can always edit it and change the game version, as well as the API version matching it.
Published versions are publicly available on the CurseForge API and website, and can be interacted with, while unpublished versions are hidden from the public eyes.
When authors want to upload a file for your game, they are prompted to tag the game version that their file is made for, and users can then filter mods according to the game version they are currently playing.