Meditation comes in many forms. From guided voice meditation, to silent sitting. There is moving meditation such as Qigong and Tai Chi, and a form of meditation that works well for those who find it too difficult to sit still for any length of time--That is, walking meditation.
Walking meditation is frequently used between rounds of seated meditation in order to help stretch the body and give it a little break before returning to the meditation mat to sit some more.
However, you can use walking meditation in many different ways. Some find it relaxing and centering to take a break from their busy work day to go to a park and walk the paths, or through the grass, to get a little more focused on the present moment.
Walking meditation can also be used as a way to move the body, yet stay focused on just one thing, such as the sound of the birds as you walk among the trees or just the sound of water if you are walking near a fountain or stream.
You can also practice synchronizing your breath with the walk. Counting "one" as you breath in while stepping with the left foot, and "two" as you exhale and step with the right. Or if you want a faster pace, you can count "one" to inhale as you take several steps, and count "two" for the next several steps. You can also place complete focus and attention on the placement of the foot. Heel, toe... heel, toe... while working to not allow any thoughts or distractions to draw attention away from this mindful task.
You don't have to force yourself to do a meditation that is not comfortable for you. Buddha was meditating in a reclined position when he died. Perhaps you've seen the statue of him lying on his side called the "Reclining Buddha". Some people who find it difficult to sit up during meditation will practice that way (i.e., lying down), and their task is to not fall asleep, but to stay awake and mindful.
So it's the same with walking meditation. If you choose a moving meditation rather than a sitting, or still meditation (such as standing meditation), the task is always the same...to train the mind to remain present and completely aware.
Thanks to World Peace Initiative for the great photo