For me, having multiple apps and windows open on my Mac's desktop is the norm. While I often shuffle through them with the Command-Tab shortcut, my cluttered desktop can feel a tad overwhelming at times.
I may go over an hour before I need to look at a particular window, yet there it is, distracting me by taking up space on my desktop.
After downloading and installing the app, look for an icon resembling a magic wand in the Menu Bar at the top of your screen. Clicking on it will reveal a list of all open apps, both hidden and visible.
Any apps that are both open and inactive will have a circle next to the name, and as time progresses the circle will fill in blue. Once time runs out (30 seconds is the default time), the window will be hidden. It's important to note that the app isn't closed, so you don't need to worry about losing work. Hocus Focus simply hides the window in the same fashion as if you had hidden it using Command-H yourself.
You can change the time it takes for an inactive window to be hidden on an app-by-app basis; simply hover over an app in Hocus Focus and move the scroll bar to the right.