This allows Cmd+Tab to behave like Window Alt+Tab, meaning you can switch between open windows as opposed to open applications. This particular behavior of the OS X and MacOS app switcher has never made sense to me, and I've always used third-party applications to fix it. Hyperswitch complains about being an expired trial every time it launches (i.e. when you reboot), but if you let it check for updates, it will let you use it anyway.
There is another option called Alt+Tab, but this app behaved oddly in Mojave when using TotalFinder, whereas Hyperswitch handles TotalFinder windows without issue.