You can get a quick introduction to the game by taking the tour.

The game is to correctly guess the color of some land cover in a country for a certain month. The colors are based on 20 years of satellite data.

The resolution of the used data means that for urban areas and water areas, the signal can be quite mixed with a lot of the surrounding land cover in the color as well.

Also keep in mind that snow hasn't been masked out. So in the winter months the land in some countries tends to be quite white.