The / path is known as the root path, which as said, is the site root.
If you think of the path in terms of folders on the system the root path is the start of the partition/drive from which you can not go up any higher in the folder hierarchy. For a web server, it is the folder the site is located in (the web server root folder is not the systems root folder).
An analogy might be a building with floors. The basement is the root and the top floor is the last path. As an aside, just like buildings can’t be infinitely high (because of physics) you may have limits to how “long” the path can be on the system (and other file path limitations).
If you mean places like freeCodeCamp, I can’t really recommend any because I haven’t tried them. But most learning is project-based because theory without practice doesn’t work.
What really matters is your own engagement and willingness to learn and seek out information. Code a little every day, build stuff, and learn new things. If you have a question try to answer it yourself first. Learn to research topics and search for the necessary information. You will find people and places along the way that you like for different topics.
It is important to stay focused, if you try to do too much too quickly, it will just be overwhelming. It is a never-ending marathon, so pace yourself. The goal isn’t to become an encyclopedia but to learn how to use an encyclopedia efficiently. Everything moves so quickly in this space that what you know now isn’t as important as how well you can adapt and upgrade your knowledge to new things.