and how to delete them?
You don’t want to delete postgres_data or uploads. You can do a
./launcher cleanup
To delete stuff that’s not needed like postgres_backup
what’s in postgres_data and uploads?
postgres_data is where your forum’s data is stored (in a PostgreSQL database). If you delete it, your forum will have no posts, no users, no… well, anything. It would be like having a fresh install.
uploads, as the name suggests, is the collection of files uploaded by your community. So, for example, if your site settings allow users to upload .JPEG images and a user uploads a photo of their cat, the image of their cat will be in uploads. It’s worth checking which filetypes your users can upload and ensuring you are happy with that, but I wouldn’t recommend that you delete existing images. If you change the site setting, you could ask that, going forward users upload images to a host (such as Imgur). A direct link to an image will embed it into the post.
Is your goal to remove discourse altogether?
What problem are you trying to solve?