This really is a cool feature. Being able to move a window around just by dragging with three fingers on the trackpad, is very convenient and I use this feature all the time.
But it is difficult to find especially since it used to be in the trackpad settings. But it no longer is. Again someone thought it would be a good idea to move the setting somewhere else. Hiding it from anyone who thought about where it might be under trackpad settings.
It's actually under accessibility settings. And there, once you get to the accessibility settings. You choose, Trackpad options. Under trackpad options there's an option to enable dragging and once you selected. You can choose, Three finger drag from a list of options.
Once you've enabled that, you just hover your mouse cursor over the title bar of any window, and then with three fingers drag across the trackpad to move that window. This saves you from having to select the window first. It doesn't seem like a huge savings of time and effort. But it really is quick and easy to drag windows using three fingers on the track pad.
Just remember to look in the accessibility settings, not in trackpad settings where you might think you'd find the option.