Sorry if this is a bad question but I find the concepts of Druidism extremely interesting as they explore natural and scientific philosophy such as the secrets of the oak and the cosmos, but I can't seem to find many explorations of these ancient concepts.
/u/searocksandtrees compiled a list of druid questions a few year ago. Between the various threads you should get an idea of what we know.
https://www.reddit.com/r/AskHistorians/comments/335yfi/how_much_is_actually_known_about_the_druids/