Junctions are like Windows shortcuts that really point at a directory rather than just pretend to when viewed with Windows Explorer. They are great for re-arranging your disk to suit your working style, or even linking between hard disks. They have been a feature of Windows ever since Windows NT(!) but the only built in tool for making them is a command line program MKLINK. This tool here provides that functionality in an easy to use GUI. Under the hood, it just uses MKLINK, which is why the binary is so tiny.  Note that despite the name the tool also supports symbolic file links, so that you can point at a single file, too.