A crankshaft pulley is a pulley which attaches directly to the crankshaft, outside the engine block, where it drives the serpentine belt. It is the component which transfers rotational energy to components such as the alternator and water pump. Its size, in relation to the size of all the other pulleys, determines the amount of the engine's power gets transferred to the various components. A large crankshaft pulley with small component pulleys will give the components a lot of power, and less of the engine's power is sent to the drive wheels.
However, if the crankshaft pulley is too small or a component's pulley too large, not enough power will be sent to the components. Because of this, there needs to be a balance between power sent to the components and power sent to the wheels. When production cars are designed, there is usually a bit of bias towards the components, because if there is just enough for the components, there won't be enough as engine power is lost with age, or if something is modified later. Because of this, a popular inexpensive performance modification for many cars is a performance crankshaft pulley. This doesn't actually add any power to the engine, it just frees up otherwise wasted power. One made from lightweight materials also helps the engine change RPM more quickly.