A brouwer is a brewer. Since most of the water was not safe to drink, most people drank beer. ‘Small beer’ only had a low alcohol percentage so it was safe to drink for children.
In larger towns, brewers were organized in guilds. In smaller towns, they operated independently. In most places, brewers had to pay tax on the beer they produced. Some of these tax records can be found in local archives or city ledgers. Also, their measures and pints were sometimes calibrated by town officials to make sure they did not cheat their customers. Some of these calibration records can also be found in the archives of the towns.
Because there were so many brewers, Brouwer is a very common last name. According to the Family Names Database of the Meertens Institute, there were over 25,000 people named Brouwer living in the Netherlands in 2007; not counting variations like De Brouwer or Brouwers.