To add to my beloved DebBEEE-H's answer:
The "hi" and the "lo" refer to impact. High impact is technically defined as any movement in which the entire body goes "aloft" even for a brief second and both feet lose contact with the floor. Low impact is techically defined as movement in which at least one foot is always in contact with the floor.
Something can be technically high-impact but fairly low in intensity, such as bouncing off and landing on the balls of both feet, or it can be high-impact AND high intensity, such as a high tuck jump off the floor. The higher up you go off the floor, the higher the impact when you land.
Usually low-impact moves are more moderate in intensity. You'll see a lot of requests for workouts that are low-impact and high in intensity, on a comparable intensity level with high-impact workouts. Usually that's not really possible. When you have a low-impact routine and you want to make it higher in intensity, you have to make the movements a lot bigger, and often that can entail its own risk to the moving joints.
A-Jock