'boost' is what a turbo creates.
a 'normal' engine, ie one without a turbo works by sucking air in, mixing it with fuel, igniting it and using the explosion to turn the crank, thus creating power.
so say you had a 5.7L V8, it would at any time have 5.7L of air/fuel in it to be ignited, and obviously more air/fuel will cause a bigger explosion, and thus create more power.
now, a turbo engine works by the turbo 'forcing' air into the engine, mixing it with fuel, igniting it and using the explosion to turn the crank, again creating power.
this is why you see alot of the turbo engines are alot smaller, ie the 2.0, 2.5, and 2.6L of the skyline, yet they can still pump out the same power as a big V6, or even a V8. say you have a RB25DET, 2.5L turbo. now whilst the turbo hasnt kicked in(ie low revs) the engine will have approx. 2.5L of air/fuel in it to ignite, and create power with, *but* once that turbo kicks in, say you are running 1 bar(14.7psi of boost, ill explain this in a bit) you are effectively pushing twice the amount of air into the engine, so that 2.5L engine is taking in as much as a say a 5L engine.
now onto 'psi', the word stands for pounds per square inch.
a normal, un-turboed engine runs at negative psi, meaning that the engine is *sucking* in air.
however a turbo engine runs *positive* psi, meaning that air is being forced into the engine, and the higher the psi, the more pressure, or 'boost' your turbo is creating.
say you had again 2 RB25DET's this time... engine A running 7.5psi of boost, and engine B running 15psi of boost.
now engine A will have approx. 3.75L of air/fuel in it at any time, whereas engine B will have approx 5L of of air/fuel in it at any time.. so the more psi your turbo makes, the more boost created, making your engine effectively larger, and creating more power.
now i better get an A for this essay...