Angles are movements. Good fighters use 2 general types of movement as far as footwork goes. Lateral front, back, left, right imagine a cross on the ground. Angular just the same but imagine an X on the ground so you will move at angles. These are both offensive and defensive. Google boxing footwork angles to see a few different things watch the boxer on the thai bags and the Muay Thai kicks vids. Pay more attention to the footwork than the punches and kicks. Usually when hitting a heavy bag you want to pull your punches but in this case it isn't a bad thing to get it swinging some and work those angles.