boost controller

its a noob-like question but what does it do?

A boost controller is used on turbo systems. At high revs, a turbo can compress more air (air pressure) into the intake manifold than the engine can safely use. The controller bleeds off the excess air , (reducing the pressure) to prevent damage (detonation) to the engine.
Older turbos used a mechanical 'waste gate', just a simple pressure switch. Newer turbos use electronic controllers that keep track of numerous factors in the engine, like revs, engine knock, temperature etc to determine the optimum boost pressure.