Answer:
"Prohibition" commonly refers to the 18th Amendment to the US Constitution, which prohibited the manufacture, sale, transportation, importation and exportation of alcoholic beverages.
Contrary to common belief, it did not prohibit the purchase or consumption of alcohol.
The Amendment was proposed by Congress on December 18, 1917, when it passed the Senate after it had passed the House the day earlier. The Amendment was ratified on January 16, 1919, went into effect one year later on January 16, 1920, and was repealed by the 21st Amendment on December 5, 1933. In the over 230 years of the U.S. Constitution, the 18th is the only Amendment ever to have been repealed.
The 18th Amendment contains only 111 words and after it was ratified only the first two sections were relevant to National Prohibition and its enforcement:
Section 1. After one year from the ratification of this article the manufacture, sale, or transportation of intoxicating liquors within, the importation thereof into, or the exportation thereof from the United States and all territory subject to the jurisdiction thereof for beverage purposes is hereby prohibited.
Section 2. The Congress and the several States shall have concurrent power to enforce this article by appropriate legislation.
Section 3. This article shall be inoperative unless it shall have been ratified as an amendment to the Constitution by the legislatures of the several States, as provided in the Constitution, within seven years from the date of the submission hereof to the States by the Congress.
However, those few words did not provide the specificity needed to permit its enforcement. For example, what was an "intoxicating liquor," what was the penalty for manufacturing it, could it be produced for medicinal and health purposes, was the quantity of illegal alcohol sold relevant to the punishment, could alcohol be produced for religious purposes, and so on.
The National Prohibition Act of 1919 was designed as enabling legislation that would answer all such questions. Commonly called the Volstead Act, it was over 25 pages in length. It was complex, confusing and difficult to interpret. However, exactly what was permitted and not permitted did not concern the tens of millions of people who chose to violate the law.