The transmission of a vehicle is part of the drivetrain, along with the engine. Its a tool to keep the engine functioning by pushing it into the right gear.
If it is a manual transmission, then that means you, the driver, will determine the right gear by using the clutch and the gear shifter located next to your seat. Most vehicles have an automatic transmission instead, which means the transmission will examine the speed of the car and the RPM of the engine to detect when to change the gears.
A continuously variable transmission is a type of automatic that has no gears-- it smoothly shifts up and down using a belt. In all of these cases, the engine must be at the proper gear to work properly, or the car will not perform properly and may be damaged.