I read the definition of barley sugar Merriam-Webster's Learner's Dictionary:
barley sugar: a clear hard candy made from boiled sugar
But what does 'boiled sugar' mean? And what does 'boiled' in 'boiled sugar' mean? What about 'boiled sweet'?
… "boiled sugar" is sugar dissolved in water and heated until it boils. Other ingredients might be added in the process and the resulting liquid, often called a syrup, is allowed to cool to produce a hard sweet/lolly/candy etc, the terms will vary depending on the region.
not a teacher