This is a new function, but I can't find any information on what it does, or how to toggle it. Help?

In the settings under "Functions" you can now Enable/Disable the Window Snapping feature with for example a TitleBar button or with a key combination. The feature itself has its own options under "Windows Snapping".
Unfortunately the snapping itself doesn't work here sometimes since beta 14. If this happens just restart DisplayFusion and it should be back again.