Meditation as I understand it involves specifically directing your mental focus to something. Sometimes breath, sometimes a visualization, sometimes an examination of a particular thought or situation.
You can do a walking meditation, where you will likewise be focused on something: one possibility being the physical sensations of walking (arms swinging, feet hitting ground), or the experience of the surroundings (light, breeze, sounds, and so on).
The difference to just walking around, or just sitting calmly, is that you are putting some effort into maintaining that particular focus, whatever it is. You will always get distracted by something else, but when you notice, you switch back. And keep on doing that for the length of the exercise.
You can do a walking meditation, where you will likewise be focused on something: one possibility being the physical sensations of walking (arms swinging, feet hitting ground), or the experience of the surroundings (light, breeze, sounds, and so on).
The difference to just walking around, or just sitting calmly, is that you are putting some effort into maintaining that particular focus, whatever it is. You will always get distracted by something else, but when you notice, you switch back. And keep on doing that for the length of the exercise.