The drivetrain is the engine of the mountain bike. You provide the fuel and these components work together to turn it all into forward motion. There are a lot of considerations and choices here, best made with some experience under your belt but our guide will help you on your way.
If it all seems a bit daunting don't worry too much, even the cheapest equipment will turn in a decent performance and allow you to work out what could work better for you later on.