A big groundwork thing is being able to "send" your horse places. As in direct them to walk left or right or turn and go places like in the trailer, through the barn, around a post, in their stall, etc. So it's like lunging but more directed towards a set goal.
Personally I really like how he explains it (minus the off topic banter)
But there are countless resources and methods that will work best with your horse, you just gotta find them!
Another good thing to teach your horse is to pick up his feet AND his front legs (for stretches, for clipping, for no reason but lack of resistance) at any time, anywhere. It's good for the farrier and if you are somewhere unfamiliar and you need to get your horse's legs out of a sticky situation or to pick their hooves after a rocky trail ride. I taught my horse by first applying pressure (leaning into his shoulder so he was unbalanced, applying pressure just behind the front leg so he brought it forward.. held it for a few seconds (before he tried to set it down) and then give him release by putting it back on the floor.