foo is a placeholder of your filename.
So yourfilename.scss, from where your css will created, need as far as I know (because I am a bloody beginner on web things) a path, for example for background
and is in html used as
If you will set background in yourfilename.html you set for example
<ion-content style='background-image: url(assets/backgrounds/hotthings.png'>
If you set it to a image it looks like
So depending where you set it und to what you set it, it will look a little bit different.
I often set things from yourfilename.ts and here I use the path like in .html will needed.
Sorry for my bad english, but my french is much more worser, okay a few not acceptable words and sentence I can speak understandable.
Best regards, anna-liebt