Don’t worry about the questions, they’re not stupid and in fact they help us figure out gaps with our product. 
The backfill is for communities who would like very old topics to be translated. All new posts and topics are translated when created or edited.
The progress chart is important for old communities like Discourse that have a lot of history that needs translating at a controlled pace.
It’s a hidden setting and default to 0 for now due to a few reasons, one of them being that this feature can be wildly misconfigured – a site may have many old posts, and the site owner may choose up to 10 or more languages. This would incur a large bill. Besides that, if the chosen LLM is the same one for other features like spam detection, getting rate limited would be not ideal.