@jfos Is correct. Being simple here, a buck converter is just stepping down dc voltage in a very power effecient way without using too much energy/heat to accomplish this. A boost dc converter will do the opposite, step up the dc voltage (voltage multipliers in computer and tv monitors are a good example of this).
It is the same with a buck/boost transformer as well. The boost transformer will step-up the ac voltage for a small application in a circuit and the buck transformer will step down the ac voltage. One exception to a buck/boost application of controlling ac voltage is also the control of pulsed dc voltage as well. Using layman’s terms here.