A barback (sometimes spelled as bar-back, also commonly known as a runner in Europe or a glassy in Australia) is a bartender's assistant.
They are responsible for keeping the bar stocked, clean, and user-friendly for the bartender.
[1] Barbacks work in nightclubs, bars, pubs, restaurants, and catering halls and usually receive a portion of the bartender's tips.
[2] Barbacks are essentially commis bartenders, and are there to simplify a bartender's job and prepare the bar by stocking it with liquor, ice, glassware, beer, garnishes, and so on.
During the evening, they also collect and wash dirty dishes and glassware, clean tables (though there is a busser for this in larger bars), change kegs, and help to pack up the bar after closing time.