RobotContainer is a class that combines the subsystems, commands, and triggers of the robot. It's basically the brain of the robot.
What to do with this Class?
This class is like a sort of hub for the robot. Here, you should write most of your non-subsystem and non-command-related code. You should also store your subsystems, commands and triggers here, rather than across different files. Having a single, central "hub" for your robot makes your code a lot more organized.
Storing Commands
As RobotContainer is the main class you'll use to store commands, you should add a variety of methods to let other classes access these commands. An example is the getAutonomousCommand() that is autogenerated with your code.
Storing Trigger Objects
ConfigureBindings() is where you'll declare most of your triggers. This method runs once, when your code starts and never again.