Treemacs is a emacs package that adds this interesting file browser view on the left of your emacs frame. Kind of like how you get with neotree or what you have in Visual Studio Code or Sublime Text or most other editors.
I was never a fan of Neotree for a number of reasons. Mainly because when I use emacs I jump between MANY different projects all the time because emacs is made for managing a ton of different shit without having to create new instance. Unlike most other editors. Especially if you use projectile. And Neotree never had any way of keeping up to date with what ever project was active in the active buffer. And the only way I found to solve it was to kill and start it again. That got old fast when you switch projects hundreds of times a day.
The way treemacs works on the other hand is that you have a similar project explorer on the left. But instead of just being one folder you can add projects to it, so if you can you manage it how ever you like it. It works great, if this is your kind of thing.
I usually don't use it. But sometimes I do. And when I do I love it. Because it is just this thing on the left that keeps up with what ever project the currently active file is a part of. The one thing I don't like about it is that the default configuration is rather big.