A camino Buff is a microfiber tube that one can wear about 10 different ways for warmth, cooling, sun protection, and hydration. It also makes a handy pressure bandage, washcloth, and hand towel.
See the home web page:
http://www.buffwear.com/
The Buff was invented some years ago by a motorcycle rider in Barcelona, Spain who wanted a skull covering under his helmet to wick perspiration in warm weather and keep his head warm in cold weather.
Once you get to France or Spain, you can obtain several Camino-centric designs, for example, a BUFF with multiple sellos, and one that looks like a WWII pilot's Escape and Evasion map of the Camino and northwest Spain.
I usually carry two, one folded in each rear pants pocket. They really are virtually weightless and their utility is limited by your imagination.
They cost about €16 - 20 each. There are cheap imitations, but why bother? The imitations are usually smaller and more flimsy.
For a few Euro more, you can obtain the 'new' Buff cap. This is a ball cap styled cap that is made from Buff material. It has foam in the brim for stiffness. I believe one size fits all. It fits my 61 cm head...
IMHO, it is a good choice for the only hat you carry if you are not walking in the really hot summer months.
Hope this helps.