Growing Potatoes

My potato plant continues to grow very well. It is such a hearty, strong looking type of plant. I really know nothing of growing potatoes. I just read the following at:
http://www.post-gazette.com/pg/08157/887403-34.stm
"When the foliage gets about 4 to 6 inches tall, it needs to be "hilled" with soil, or mulched up to about the bottom of the leaves. This keeps the potatoes from exposure to sunlight, which turns them green. Once the foliage dies, the potatoes can be harvested."

Apparently it's time to hill my potato plant as I know it is 4 - 6 inches tall! So, what exactly does "hilling" mean??
In researching that I came across a gardening help site (link follows) that gave good directions on how to hill your potato plant (can be found in the article under the section titled "planting in rows")

http://www.thegardenhelper.com/potato.html