In Warhammer 40k, there are two planes of existence: the material plane, and the Warp. Formerly known as the Realm of Souls, tens of millennia of war and death and suffering have transformed it into a nightmare world akin to Hell. The Warp reflects the collective unconscious of nearby conscious beings. For example, the warp near modern day earth would be a heavenly stock market run by Supply Side Jesus, because that's what most modern humans believe in. Our fundamental beliefs about the world, either religious or otherwise, manifest in the Warp as spirits. The Warp also has the ability to influence the material plane through basically magic. Powerful individuals called Psykers have the ability to manifest their will directly in the Warp instead of unconsciously, and cause that will to affect material things and thereby cast spells. Everyone has some level of psychic potential, but usually it takes a lot of people believing in something as strongly as a religion.

Orks are a genetically engineered bioweapon that has decayed over tens of thousands of years from the ultimate soldiers, to a bunch of murderous football hooligans with cockney accents. Orks have stronger median psychic abilities than humans, but they don't have exceptional psykers who can do fancy things on their own. Instead, the collective will of all the Orks in an area forms a WAAGH field which enforces their collective beliefs upon reality. This is what makes their crude technology work. There are also Ork shamans, who can't use psychic powers on their own, but who can direct the WAAGH field to cast spells like a human psyker. A shaman is only as powerful as the ambient level of WAAGH nearby. No Ork army, no magic powers.

Note that humans are capable of the same thing, their abilities are just much weaker per human. The reason everything always sucks in 40K, the reason suffering and death are so normalised, is because trillions of humans believe in suffering and death at a fundamental level. It's a self fulfilling prophecy. Humans created three of the four Chaos Gods. Humans are the ones choosing, on a subconscious level, for everything to keep getting worse and more miserable. Orks, on the other hand, are actually having a great time. Orks are genetically programmed to love fighting, so the human belief in endless war has created a great environment for the Orks, who are happy to do their part to keep the party going.