It's when someone wakes up and finds out they're in another world that's not their own. Usually, it's a magical faraway land with dragons, wizards, princesses, the whole shebang.
Sometimes, the MC becomes a chosen one or they have to now fight the big bad or something. I feel an example most people are familiar with (assuming they don't consume a lot of anime/manga) is The Wizard of OZ. Like, it's not entirely the exact same thing, but it has similar element to it.