Adult animation is like a cartoon but not for kids, it's for grown-ups. Just like how grown-ups have their own serious movies and TV shows, they also have their own cartoons. But unlike kids cartoons, adult animation has more mature themes like violence, nudity, and bad language.
The cartoons that grown-ups watch are usually made for them because they can handle more serious topics than kids. Some of the most popular adult animated shows are Family Guy, South Park, and Rick and Morty.
So, to sum it up, adult animation is like a cartoon for grown-ups that has more "grown-up" themes and humor.