In the UK it is called “Jam Sugar” and in the US “Gelling Sugar” but both are essentially the same: sugar with added pectin and citric acid. Sugar, pectin and acid are all required to make jams and jellies set correctly. As jam sugars are formulated with the right amount of pectin, the jams and jellies you make using them are much easier and a lot firmer. You can find jam sugar in most supermarkets.
Kind regards, Véronique