It's a serving of sugar in the form of a solid dice, an equivalent of "a teaspoon." Until the 19th century, sugar was marketed in the form of solid bars and the portions were cut to be served on trays or bowls along with tea or coffee, but the Swiss Jacob Christoph Rad came up with the idea of selling it already fractionated into small cubes of about 15 mm on a side, so he began production at his plant in Bohemia, which then belonged to the Austro-Hungarian Empire and today is Czechia. The first clods were dyed with a red vegetable dye, and legend has it that it was a tribute to Rad's wife, who when she hurt herself with a knife breaking a sweet bar, was the one who inspired the idea of selling it pre-cut.
