Warhammer 40K or 40,000. Is the name of the sci-fi franchise warhammer.
The 40K signifies the year, 40,000+ AD(the main bulk of the story has moved into the 42nd AD but ppl still call it 40k)
In Warhammer, everything sucks for the people living in it. Humans for example; are in a religious-totalitarian-fascist-monarchical-dictatorial-authoritarian regime, where cruelty and violence rein supreme. The humans have to struggle against other alien and magical factions(Warhammer is a sci-fi fantasy as well) where everybody and their dead (possibly alive) ancestor is looking to dominate the galaxy for themselves.
Warhammer is grim, dark, brutal and invented the genre of 'grimdark'. It's about the struggles of each race against their 'doom clock' (a metaphorical realization that all of them will die)a clock that beats specifically for each race who have their difficulties, but the doom clock also has disasters capable of ending the galaxy and completely annihilate all the races and factions in Warhammer.
Another thing about Warhammer is how bullshit it is; the motto in warhammer is "Nothing is overpowered if everything is overpowered". The whole universe has utterly insane power levels and often doesn't make sense with how nonsensical some things are.
Like let me give you an example; the Imperial Guardsman who number in the trillions with full-body flak armor capable of taking a few laser gun shots, and wielding a laser gun (lasgun) incapable of jamming, can blow off limbs with every hit, shoot 100 shots in a minute, virtually infinite ammo and recharge from 2 hours of solar energy— are called flashlights in the warhammer universe. Their such a weak weapon that their called 'flashlight' by the fans and even those inside the universe.
The Imperium (humans) basic frigates are 2-4 kilometres long(the size of super dreadnoughts in other sci-fi), hundreds or thousands of years old, have entire generations of human live and die on it, and the frigate's basic construction materials are things like adamantine and other extremely potent fictional metals.
And this is just the human faction; it gets waaaay fucking crazier with other factions. The Necrons, Eldar, Chaos, Tyranids, and even Tau are not fucking around; all of them have their unique absurdities.