An introduction to assets
The assets of your theme are its images, javascript, font and css files and the settings.json file where theme settings are defined.
Assets are contained within the assets folder and its subfolders (visible when editing the theme in advanced mode).
The settings.json file is contained within the root.
Best practice is to keep any third party css or javascript within the assets/vendor folder.
Image folders
You may notice a few different image folders, as shown on the example above.
There's no significance to this structure.