Hussite Reforms is a technology in Age of Empires II: Definitive Edition - Dawn of the Dukes that is unique to the Bohemians. Once researched, Monks and Monastery technologies have their gold cost replaced by food.
Hussite Reforms is a very peculiar technology, in a way it works like the "trash unit making" unique technologies like Kamandaran, Corvinian Army, and Forced Levy by turning the Monk into a "trash unit" that costs 100 food now, and at the same time it changes the costs of every Monastery technology into food. This is the only technology in the current metagame that changes the price of other technologies of the same player (Huns' Atheism used to change the cost of Spies/Treason but that effect was removed). All of this of course makes Monks a much more viable option at the late game especially when gold runs out.
In a trash war scenario, Bohemians Monks with Hussite Reforms can counter small armies of Halberdiers, Skirmishers, Genitours, Magyar Huszars, and Malay Two-handed Swordsmen, and can be countered by the Scout Cavalry line and Persian Crossbowmen with Kamandaran. The usefulness of Hussite Reforms is also very noticeable when facing civilizations that has units with low conversion resistance, such as the Elephants or heavy cavalry. Despite how cheap Monks become with Hussite Reforms, they still require good micromanagement in order to use them offensively.
Usually Bohemian players have to research Fervor and Sanctity in Castle Age because these two technologies benefits Villagers for them, as well as in most games the player has to train at least one Monk in Castle Age for Relic hunting, so normally a player that uses Hussite Reforms must take in consideration those gold investment made previously. Considering the gold cost of the technology it starts to pay out after the player trains 5 Monks and/or 2 cheap Monastery technologies or one expensive Monastery technology (like Faith), of course the player must research Hussite reforms as soon as possible if he or she is planning to mass Monks in order to get the most of the technology.
Considering Bohemians are a prominent gunpowder civilization and has the Hussite Wagon and Houfnice as their unique units while having fairly good Arbalesters, they can pair their trash Monks with either of these units from early Imperial Age on, as those units require Wood and Gold, while Bohemian Monks will only need Food.
The Bohemian Reformation (also known as the Czech Reformation or Hussite Reformation), preceding the Protestant Reformation of the 16th century, was a Christian movement in the late medieval and early modern Kingdom and Crown of Bohemia striving for a reform of the Roman Catholic Church. Lasting for more than 200 years, it had a significant impact on the historical development of Central Europe and is considered one of the most important religious, social, intellectual and political movements of the early modern period.