A manual boost controller (MBC) is a hand-operated device that modifies the amount air pressure from a turbocharger that is allowed to enter the intake manifold. By regulating the amount of air pressure, the total amount of available air is also being regulated. A turbocharger is powered by the engine’s exhaust gas and is limited by a device on the turbo called a wastegate. This device prevents the turbo from generating extreme air pressures that would damage the engine. An MBC simply allows the user to make fine adjustments to the wastegate, which changes the amount of boost from the turbo.
For more info, see Car Tuning: Turbo Tips