Chess-playing computers have been around for years. I had one when I was a kid.
Chess is quite complex: it has about 9 million possible moves. But the game 'go' has more moves than there are atoms in the universe. So it's a huge development that a Google AI program has beaten a human champion. It got so good by observing and learning from millions of games.
Experts thought that this wouldn't happen for another ten years. It might seem like just a game, but it's a step towards artificial intelligence that can solve more complex real-world problems.