It says in the rules that energy can come from the altitude of the robot’s center of gravity, what does that mean?
It means that you can have a mechanism that powers itself via gravity. IE: you could have a giant rock with a rope tied around it which drops and powers something by pulling on the rope as it falls downward.
Or you can roll down a ramp if there is one on the field somewhere. They wouldn’t penalize you for rolling down a ramp!