Apps · The Apps window and launcher
What does the Hide running switch do in the Apps window?
Short answer: Hide running removes currently open apps from the Internal list. It's on by default because running apps can't be moved; quit the app or turn the switch off to see it.
Hide running filters out apps that are open right now. It's on by default because an app that's running can't be moved anyway, so it just keeps the list tidy.
If you're looking for an app you have open, either quit it and refresh, or turn the switch off to see it in the list (it will show as blocked until you quit it).