It may not be explicitly stated in Denis Villeneuve's Dune, but fans of Frank Herbert's book know that there is a very good reason why everyone fights hand-to-hand and uses swords and knives instead of guns and projectile weapons. While it is not something the audience needs to know before seeing Denis Villeneuve's Dune, it still enriches the experience to understand how Dune's technology works. In the Dune universe, swords and knives don't just lend themselves to stylish fight scenes, they have a practical reason for being the weapon of choice.
Frank Herbert's Dune has been a much-celebrated science fiction novel since it was written in 1965, as it's the first book to win both the Hugo and Nebula awards. Partly due to its length, the novel has proven notoriously difficult to adapt to screen, with Jodorowsky's Dune as a famous failure, and both the 2000 Syfy mini-series and David Lynch's 1984 Dune ranking as unsatisfying realizations for many, making Denis Villeneuve's version unusually successful by comparison. One thing that Villeneuve's version got right is the nature of action in the Dune universe.
The main reason there are so few projectile weapons in the Dune universe is because there is a cheap and readily available personal energy shield that wraps around the body of its wearer and protects them from nearly all projectile weapons. It's small and portable enough to fit on a bracelet, and can be turned on in an instant. All soldiers and all members of a noble house carry one. The Holtzman Shield was a turning point in the technological advancement of the Dune universe and its timeline. The invention of the shield made ranged warfare nearly obsolete.
There are a few ways to penetrate the Holtzman Shield, but the most practical method is by a slowly moving sharp object like a sword or knife. The Holtzman Shield, as Gurney Halleck briefly explains to Paul Atreides during their sword fight training, repels fast-moving objects but allows something moving slowly to get through. Presumably this is so the wearer of the Holtzman Shield can still manipulate objects and breathe, and have a freedom of movement not restrained by the rigidity of the shield. Villeneuve's Dune distinguishes this well during all of Dune's character deaths by signifying fast hits on an active shield with blue sparks, and when something penetrates a shield with red sparks. There are projectile weapons designed specifically in the Dune universe to penetrate shields by moving slowly. It is only a slow-moving and spinning projectile that penetrates Duke Leto's Shield.
There are a few projectile weapons that are still used in Dune despite the Holtzman Shield, and Villeneuve's Dune displays them well. When the Harkonnens attack House Atreides they use several missile-like weapons. Fremen also use different "slug throwing" and other archaic weapons in their raids. These projectiles would all bounce off a Holtzman Shield. Dune has a lot of terminology, but one ranged weapon to know is the "lasgun", a bright blue beam weapon that cuts through anything. In Villeneuve's Dune it comes up twice, once when Duncan Idaho is making his escape, and once when the sardaukar are breaking in to capture Paul and Jessica. Under the rules of Dune's technology, if a lasgun hits a Holtzman Shield, it generates an atomic explosion somewhere along the line of fire. This makes both uses in Villeneuve's Dune represent acts of desperation.
Villeneuve's Dune leaves the relationship between the Holtzman Shield and projectiles unexplained and the politics of the Spacing Guild unexplained — even the meaning behind Dune's ending needs to be explained. This is probably for the best. At two-and-a-half hours the movie had time, but distilling Frank Herbert's story into a movie is notoriously difficult for a reason. Fans of the book will know the relationship between the Holtzman Shield and projectiles, and there are enough clues in Villeneuve's Dune that repeated re-watching will just make for a rewarding experience.