Applications that are in fullscreen like a YouTube video or game work just like on any other DE as in they take the entire screen and cover any UI elements. They can however still be moved to other tags or you can switch to another tag to still retain multitasking functionality.
This feature allows you to temporarily put a window into fullscreen mode without impacting any of the other windows or its actual position and size. It can be activated using Super+Ctrl+F and using it a second time restores the window to its normal behavior.
For example you could be in the grid layout working with lots of windows but would like to examine the contents of a window using the entire screen. Instead of switching to the monocle layout and resizing all other windows as well the tempfullscreen feature can be used to enlarge and then shrink the window back down.
Fakefullscreen allows you to resize and move fullscreen windows. The application will still think it is in full screen. This can be used to move applications that would otherwise be stuck on the monitor they launched on or to correct applications that behave incorrectly.
To toggle fakefullscreen, press Super+Shift+F. If the application is not already in fullscreen, this will not have any noticeable effect. But as soon as the application goes into fullscreen, it will not cover the entire screen, but can still be put into layouts, moved around in floating mode or even resized. Some applications will not like being resized while in full screen mode, so be careful with that. Fakefullscreen can also be activated for applications that are already in fullscreen mode. Pressing Super+Shift+F a second time will deactivate fakefullscreen.
Applications for this are forcing games that like to appear in fullscreen and on the wrong monitor to behave and make video players or websites that don't support picture-in-picture functionality "support" it by entering fullscreen mode and then floating them around with fakefullscreen.