Because of their size and the heavy glass components inside, large lenses are subject to something known as "zoom creep" where they have the tendency to extend or collapse under their own weight when pointed upwards or downwards. In some situations the problem can be easily solved by just holding the zoom ring, but other times you might need a specialised tool.
Usually that specialised tool is nothing more than a couple of elastic bands wrapped around the lens to hold the ring in place, but let's be honest here, that solution is far from aesthetically pleasing. So for just $US5 the Lens Band, made from high-grade silicone rubber, does exactly the same thing, but in a variety of different colours. Including black if you want to use one but don't want it to stick out like a sore thumb.
It's designed to work with lenses from all the major manufacturers, including Nikon and Canon, but as far as I can tell it's a "one size fits all" kind of deal so you don't need a different one for every lens in your kit. Of course, if you've supported any kind of charity over the past three years, you're probably already wearing multiple Lens Bands at this very moment.