Every copy of Pokemon HeartGold / SoulSilver comes with a Pokewalker. The bundled accessory is a pedometer of sorts, reminiscent of Pocket Pikachu.
You upload one of your Pokemon into it, choose a course, and then you take it for a walk or, if you’re like some people, you attach it to a fan or something and just let the “steps” rack up. This increases your Pokemon’s friendship as well as its level. However, the main purpose of walking so much is to increase a parameter called “watts.”
Approximately 15 steps equal one watt. With three watts, you can use the item finder, and with ten watts, you can do Pokemon Training.