Small beer is a beer that contains very little
alcohol, perhaps less than one percent. Sometimes unfiltered and porridge-like, it was a favored drink in
Medieval Europe and colonial
North America. It was sometimes had with breakfast, as attested in
Benjamin Franklin's autobiography. Before public sanitation
?, water-transmitted
diseases, such as cholera
?, were a significant cause of death. Because alcohol is toxic to most water-borne pathogens, drinking small beer instead of water was one way to escape infection.
Small beer is not made in any significant amount today, though a similar product called non-alcoholic beer (which actually has a little alcohol) is quite common.
Metaphorically, small beer means a trifle, a thing of little importance.