You use this plugin in Godot version 3 to save a node's mesh as an STL file. This is very useful if you've generated terrain for instance and want to 3D print or CNC machine it.
Download Save as STL.zip
Unzip it, it's a Godot Project which you can open and play with. To use it in your own project copy the folder save_as_stl into your project's res://addons/ folder.
Go into Project > Project Settings and select the Plugins tab. Save as STL should show up, or click Update. Make sure the Status is set to Active. Close the settings.
Now you can select a Node and then go to Project > Tools > Save Selected Node's Mesh As STL. A File Dialog will appear and you can choose where to save your STL file.
Watch the Video