Minecraft is a game about breaking and placing blocks. At first, people built structures to protect against nocturnal monsters, but as the game grew players worked together to create wonderful, imaginative things.
It can also be about adventuring with friends or watching the sun rise over a blocky ocean. It's pretty. Brave players battle terrible things in The Nether, which is more scary than pretty. You can also visit a land of mushrooms if it sounds more like your cup of tea.