The File Sidebar
There is no prominent button in the UI for toggling it on. You can toggle it by pressing Command/Ctrl+B or selecting view: toggle sidebar in the command palette or View menu.
The file sidebar is now a stable feature, and it displays the files and folders in the current project(if any open). You can click on a file to open it in the editor, or click on a folder to expand it and see the stuff inside.
Note
Symlinks are currently shown with a link icon and may not be opened for stability reasons. That feature might be added in the future.