It’s not a sugared plum, in case that was your guess. Historically, sugarplums were just little sugar-coated whatsits (what were known up until the early 20th century as comfits). Maybe a caraway seed, a nut, a bit of a cinnamon stick, a raisin, or other little piece of dried fruit. Candy makers would cover these in up to a dozen individual layers of sugar syrup, the end result being something analogous to a sugared almond.
Some food historians will argue that sugarplums were actually the size of plums, but I personally find that a stretch. Even now, none but the most resolute sweet eater would be willing to take on a hard candy the size of a golf ball. Rather they would have been quite small, about the size of pearls. And in fact people did eat them out of small packets, boxes or tins like we eat M&M’s or breath mints (some were actually called “kissing comfits”).
They survive today as dragées, the little gold or silver balls that adorn Christmas cookies and wedding cakes. Sprinkles (or what are known in Britain as hundreds and thousands) are also part of the lineage.