what is attack noob?

Hi,
You can click on any of the option names/labels in the UI and you'll be presented with a description of how that option works.

For "Target Noobs" it will show you

"If this is set to false then npcs from this profile will not attack 'noob' players.
This setting uses Rust's built-in sash system and is automatically disabled if NoSash plugin is installed."

The vanilla sash system, as far as I know, works on whether or not you've handled a weapon or tool, excluding the default torch and rock.
Once you have handled anything else - A gun, knife, farming tool etc, you are marked as no longer being a noob.

Essentially this setting will protect newly spawned players from otherwise hostile npcs.