You know when a mainstream book goes really big, and it’s something that uses SF tropes. Like, for instance, if someone writes a books with vampires and then acts as though urban fantasy doesn’t exist. Her fans read it. They say, “OMG! These vampires that she’s invented are the best thing ever!”
And then they run across Dracula and are all, “Hey– this Stoker guy is totally ripping off vampires. And they don’t even sparkle.”
Or, say that there’s someone writing dystopian SF, who then says, “I don’t write science fiction” or disparages it as “”talking squids in outer space.” Then goes on to get all the interviews and the awards and completely eclipses the people in the SFF community who are writing dystopian SF.
And your head explodes with fury and outrage?
That’s your culture.
Now extend the metaphor and imagine that your culture has been systematically oppressed for centuries. That gives you the general idea what cultural appropriation is.