A microbrewery, or craft brewery, produces a limited amount of beer. The maximum amount of beer a brewery can produce and still be classed as a 'microbrewery' varies by region and by authority; in the US, it is  a year.
The concept of a microbrewery is tied to the amount of beer they produce. The definition can vary depending on where you are, but in the United States, it's based on an annual limit.