Emacs is literally the best self-documenting system in the world - zero exceptions. You can easily see the documentation of every function which usually explains what it does and what each parameter is for. And in case you are not satisfied with that, you can quickly jump into the definition and see the code for yourself.

„C-h f“ (help on function) or „C-h k“ (help on keybinding) at least gives you some quick information on any function in Emacs. I also recommend „which-key.el“ showing you which commands are bound when you typed in an incomplete key combo. You probably also want some improved completion engine like ivy/counsel showing you the key bindings in the command list after hitting „M-x“.

This way I discovered a lot of helpful stuff.