The movie starts with an introduction to the Kim family, the parents and two teen-adult children living in a small, lousy apartment, barely making a living folding pizza boxes. The son is introduced to a very rich family, and becomes the English tutor for the rich family's daughter. He casually suggests "Jessica" - a girl he claims not to know, but who he says has a good reputation - as a tutor for the rich family son. And so his sister is hired. And then the father. You get the idea.
This sounds like a movie about scam artists - but it's by Bong Joon-ho, and it's Korean cinema: don't assume you know what it's about. As you've probably heard, there's a twist. When you start the movie, you're likely to have no idea what the title means. You can guess: you'll be wrong. By the time the movie ends, you'll have a very clear answer.
I've only seen one of Bong's previous films, "Snowpiercer." Which I loathed, in opposition to almost everyone else on the planet. This one is better, but my personal biases (I don't usually like creepy movies, and this is way creepy) mean that I don't love it. Although I admit it's intriguing and memorable, in a horrible way.