A joule is a measurement usually used in chemistry or another form of science. It usually measures some energy. Energy is defined as work done on something when force is executed on the item. Different effects can be applied to an object. One is to push or pull something. Another could be heated. Heat changes things. The heat will melt a substance.
This can be measured by the number of joules of heat it took to melt something. It also depends upon the weight and amount of the material being dissolved. In this case, heat is needed to melt a substance that weights 7.5 grams at its melting point. The temperature of fusion for an element is one hundred twenty-two joules per gram. If you multiply one hundred twenty-two by seven and a half grams, it would be 915 joules per gram.