It's certainly hard to nail it down properly in less than a 5000 word essay (maybe 10,000) but if someone asked me for the key points it would be:
Set 40,000 years in Earth's future. Humanity controlled by a religious, authoritarian, fascist Imperium that worships a corpse who used to be a superhuman. Main faction are the Space Marines, who either serve the Imperium, or the main antagonist Chaos which comes from a hell dimension. The Imperium is also beset on all sides by multiple alien races, across the whole galaxy.
Also, there are flying cherubs, because somebody once invented AI, back when the world was basically Star Trek.
And there are cathedrals on legs.
And sentient fungus that thinks painting something red makes it go faster. Which is correct.
The Rogue Trader CRPG that was released last year is also a great entry for people who don't know much about the universe. There's a lot of description and pretty much a built-in encyclopedia. Plus, Rogue Trader is more like Warhammer 40,000 exploration. Think of it as a grimdark Star Trek.
I have to be honest. I loved the graphics and aesthetics but I found the campaign to be boring and very repetitive. I didnt like much this game unfortunately
Oh honestly, I found it an absolute slog and gave up on the 8th mission. Just shooting a bunch of fodder for 90% of the game got really boring and the story/dialogue were dreadful.
Some of the voices also, like the Archmagos, painful on the ears.
But, I'm happy it's doing well, I like the message that it sends about short, linear games being in demand.