What does "./" (dot slash) refer to in terms of an HTML file path location?

You can use the following list as quick reference:

   /   = Root directory
   .   = This location
   ..  = Up a directory
   ./  = Current directory
   ../ = Parent of current directory
   ../../ = Two directories backwards

Useful article: https://css-tricks.com/quick-reminder-about-file-paths/

/ means the root of the current drive;

./ means the current directory;

../ means the parent of the current directory.