Its indie I think, but it has single player and multiplayer

Not MMO if you count "massively multiplayer online" as meaning "a crapload of players in the same world". The world is massive though (theoretically infinite, though things get weird when you go far enough from the spawn point - "normality" still covers about 8 times the surface area of the earth).

Minecraft is very hard to explain. It's a sandbox game, set in a dynamically generated infinite (sort of) world. It has no goal, other than to play with the game mechanics, but there are a lot of those, and it is easy to lose track of time playing it.

You can build stuff yes, but for that you need resources, and to get those you need tools. In order to make those, you need more resources and infrastructure, and it all sort of evolves from there.

There are monsters, but they are not particularly dangerous, if you know what you are doing (but very much so if you don't), and they only spawn in dark areas. This means underground, or outside when it's night. There are also peaceful animals, which are sources of some types of resources.

One thing about the game. You will need to craft a lot of different items in order to do various things. No crafting recipes are taught in the game. You will have to find those online, for instance in the Minecraft wiki. If you find this a game breaking annoyance, then Minecraft is not for you, at least not yet. The game is under continuous development, and new features are added periodically, so maybe crafting recipes will eventually be available in-game.