Berthold was a priest in Palestine. There were a lot of people living as hermits all over the region, each doing their own thing. Berthold got them together to live in a community on Mount Carmel. So, he helped start the Carmelite Order, which is still very large and active today. He was the first superior of the order, which means he created their rule of life.
It's Palm Sunday for Carmelites and for the whole Church! When you receive palms at Mass today, remember these are sacramentals and need to be treated respectfully. Tuck them behind the crucifixes or icons hanging in your home, Here's the whole story at Catholic Culture - plus some links for how to weave palms or fold them into crosses.