Jelly is a dessert made with sweetened and flavored gelatin. It can be made by combining plain gelatin with other ingredients or by using a premixed blend of gelatin with additives.
The synonym Jam is UK English for what is commonly known in US English as a fruit preservative. It's similar to Marmalade and often made with strawberries. Despite the term "jam", the graphic used in the game shows a moulded gelatinous dessert, rather than a jar of preserved fruits.