How to use a relative path in a parent folder in electron app?

How do I use a relative path without specifying a specific drive in my electron app? I need to use something like SRC="/FILE.HTML" so my app will work on any drive installed not just a specific location. I know ./ and ../ go to previous parent folders but how can I go a few more directories back? I’ve seen things like %appdir%/MAIN DIR/SUB DIR/FILE.HTML, however I’m not sure how that works? Maybe I can setup some kind of variable to use in my path like that? So far I’ve only been able to use direct paths to go to files in previous folder which constricts my software to a specific drive.

Example: MAIN DIR/SUB DIR/FILE.HTML