The Imperium of Man is a space empire run by Earth (or Terra). It was formerly ruled by the Emperor, an autocrat with superpowers, but because of a combination of him being a terrible dad, his failsons being unable to communicate, and just fuckery from evil gods throughout the universe, he was brainkilled by one of his sons, and half of his sons turned evil. But his corpse still has JUST enough superpowers within that it's able to power transportation throughout the galaxy

Now, officially, his corpse still rules the Imperium, but now, there's a state religion devoted to him, and multiple bureaucrats pretend to speak for him, and of course, everything he wants is in the interest of the capitalists and the imperialists. It's implied that one guy sneezing wrong in his throne room would destroy him, making the galactic transportation impossibile, thus destroying the Imperium, if it's not destroyed by either it imploding on itself or destroyed by an external forcem

Now, it's supposed to be an obvious satire of fascism. The Imperium sucks. The one thing it's supposed to do, destroy all external threats, it's terrible at. But fascists either don't see the satire or don't care.

Now, don't get me wrong, the writers are not completely innocent here. First off, the marketing is almost exclusively Space Marines looking awesome. And many of the writers can't decide between three different, contradictory messages:

1. The Imperium is a shitshow, and it's all because absolute power was given to an autocrat and his failsons.

2. The Imperium is pretty bad, but it's necessary for mankind to exist.

3. The heroic Space Marines save the humans from danger.

This obfuscates the satire pretty hard. Another thing is obsession with making it "gramdark enough". Like, originally, the Tau Empire was another faction that was as close as 40k had to good guys. It was not fascist. It wasn't perfect, but it had a good place in the story. It had the message "even though the Tau Empire isn't perfect, it's proof you don't need to be fascist to survive the universe of 40k". But people complained it wasn't grimdark enough, so it was made more evil (personally, I disagree that the concept is not grimdark, it makes the universe seem bleak that the one good faction stands no chance against the Imperium). But here's the thing: Fascists claim they like grimdark. They do not. What they think grimdarkness is is cruelty to the others. It's not. Probably the most famous story described as grimdark is George R. R. Martin's A Song of Ice and Fire, and it's not just mindless cruelty. It's a highly nuanced story in which you look into all sides. Fascists don't like that.

(As a side note, a fun way to make people who say the Imperium is fine because they're justified is to remind them that the Dark Eldar are much more justified.)