Imagine you've been abducted by aliens, knocked unconscious, and returned to Earth randomly. You wake up on a random street and have to figure out where you are. That's basically how Geoguessr works. It places you randomly anywhere on Earth with Google Street View and you have to figure out where you are and drop a pin in the map provided. The closer you are to correct, the higher your score. I think the highest you can get is somewhere around 6500. Each game consists of five sessions, so I've been treating a score of 30,000 or better as respectable.
I played this all weekend with my 7-year-old and we had a great time spotting languages and identifying climate zones. We cheat like crazy with another map open in a separate tab. Then he goes to bed and I keep playing, it's that fun.