The Fremen are the original and supreme rulers of the planet Arrakis in the Dune saga. Men used to living in the desert, obedient to his commandments and able to travel long distances by running or riding the fearsome Sandworms. With the release of the Dune movie, why not take the opportunity to talk about it? Frank Herbert's work has inspired other great franchises, one of them all Star Wars, and their modernity even after some time still makes them noteworthy today. The parts full of spoilers for those unfamiliar with the saga will be marked as such and absolutely avoidable.
The Fremen are described as descendants of the Zensunni, interstellar nomads followers of a particular philosophy, which led them to wander from planet to planet until reaching arrakis, the only ones to survive on the inhospitable planet thanks to the rigid inner discipline of their creed. Trained in guerrilla warfare since childhood, they are considered among the best fighters in the universe, the only ones able to stand up to even the ferocious Sardaukar Emperor (the fact is not accidental: Salusa Secundus, the inhospitable world where the Sardaukar are forged, was one of the stages of the Zensunni exodus; and Arrakis proved to be a much more severe gymnasium than the prison planet of the House of Corrino).
Constantly living on Arrakis, particularly in the desert, Fremen tend to have very dark, almost sun-baked skin and their constant exposure to spice makes their eyes blue. Given the nature of their culture, they are more or less all athletic and capable of enduring long-distance desert runs. They manage to survive by taking in little water a day and recycling what they lose.
The Fremen are organized into a matriarchal society that sees them divided into tribes, each identified by their home, a rock formation in the Arrakis desert known as the sietch. Each of these tribes is ruled by a Naib, which in the Fremen language means "Someone who has sworn never to be taken alive by the enemy", and the brutal ritual to become one identifies the warrior society of the people of Arrakis: to become Naib one must kill in a duel the previous one.
A key aspect of Fremen life is, of course, water. Given the scarce availability of the same on Arrakis, the Fremen wear suits that preserve all the fluids that the human being naturally loses during the day. The Fremen creed states that water belongs to the tribe, including of course that of the human body. When someone dies, the water inside the body is completely extracted and delivered to the Naib for good use. The only case in which this does not happen is the sacred ritual of the duel, a fight that takes place without suits whose winner is entitled to receive the water of the loser as compensation for the one lost through blood and sweat.
The Fremen worship the Sandworm, called Shai-Hulud, as a desert deity. He is both creator and destroyer and thanks to his pups the Fremen can draw on particular poisons to perform some of their most important rituals. It is also the custom of the Freme that the blind are left in the desert to die, so that Shai-Hulud can dispose of them and, presumably, put an end to their suffering.