Basically no web page can live without the assets, which are all the files which are not HTML and belong to the web pages. All these must reside in this directory, one subdirectory level within this directory is permitted. Of course also assets (e.g. CSS files) can contain Liquid code - such files must be marked by adding the file extension '.liquid' to them (e.g. main.css.liquid). Beside CSS files also SCSS files are supported and are automatically compiled when needed.