Speakeasies were popular in the US
during the 1920's and early 1930's, during the Prohibition
era, during which time alcohol
was banned. Since the demand for alcohol didn't stop once the laws were enacted, people resorted to illegally purchasing it, and a lively underground bootleg
er trade rose to supply the goods.
A Speakeasy is a saloon, although kept hidden, so customers could drink alcoholic beverages without fear of getting arrested. These were often in basements of buildings, hidden so they could not be easily discovered.
To uphold security of the establishment, many places would require you to give some kind of password to the man guarding the door. You either had to be a trusted client or have connections to enter many of these places, letting people know that "Walt sent me" for example would let you in.
Inside, it was a regular bar. However, raids became common, as Federal Agents would storm some of these speakeasies and catch all the patrons and barowner red-handed. Speakeasies would then, instead of serving in beer mugs, serve the liquor in teacups with saucers, so in the event of a raid, the people could look law-abiding. I don't know if that improved matters. If caught, the offenders would do jail time, the speakeasy would be smashed, and the alcohol would be dumped into the sewer or the river. The movie The Untouchables captures this well, as agents storm in yelling "This is a Raid!" and hacking the tables and chairs apart with an axe and overturning barrels of booze.
Many times, people would bribe police or government officials to overlook the speakeasy. Gangsters and bootleggers like Al Capone got a good deal of their income from these places, and other activity like gambling and racketeering took place in these speakeasies, away from the watchful eyes of the law.