Water
Water is a precious resource, especially on the arid planet of Arrakis. Control over water resources is one of the key factors in the struggle for dominance among the Great Houses.
Lore
In the past, Arrakis was a green planet rich in water. Desertification was caused by the introduction of sandtrout (the larvae of the Sandworms). Those larvae created the arid environment necessary for the survival of adult Sandworms, for whom water is lethal (an excessive amount of water would kill the Saandworms, bringing an end to the production of the Spice forever and destroying the Imperium's economy.) The sandtrouts will actively seek out ree water sources and move then underground to create the arid and dry environment ideal for the adult worms.
At the planet's northern poles, there are perennial ice caps. These represent the main source of "legal" and commercial water for Imperial cities such as Arrakeen. The Water Shippers control the extraction of water from the polar ice caps and its distribution via ornithopter to commercial centers.
For the Fremen, water is even more precious though, as they sees the human body primarily as a reservoir of water.
When a Fremen dies, his water is extracted and returned to the community (sietch) through body distillers; from that moment it belongs to the tribe, not to the individual. Even wasting water by crying is an act of the utmost respect, known as "giving water to the dead.". Spitting in front of someone means voluntarily giving them a portion of one’s own life-giving fluid, which is considered a sign of honour and respect.
To collect the water essential for survival in the desert, the Fremen have developed advanced technologies such as Stillsuits and Windtraps.
In-Game