The Ilio-tibial band, a.k.a. the ITB, is a long band of fascia. What is fascia you might ask? It is connective tissue (like tendons and ligaments) that surrounds muscles, bones, and joints, providing support, protection to the thing it surrounds. I usually explain it like this - you know when you take the skin off a chicken breast and there is a thin layer of something that is not skin and not flesh - that is the fascia (sorry to all you vegetarians out there). The ITB is a super thick layer of fascia in the body, second only in thickness to the fascia surrounding the skull. It runs from the pelvis to just below the shin on the outside of the thigh.
But what you really need to know is that fascia is like shrink-wrap. Once it gets tight it tends to stay tight and this can cause pain and injuries. If your ITB is tight you can get serious hip or knee pain. If you run or hike, especially in hilly areas, you should stretch your ITBs regularly to help maintain healthy legs.