Is this an old textbook?
J is the symbol for a quantity called 'The mechanical equivalent of heat'.
Both heat and work are forms of energy (as are all the other terms in the First Law) but we need to ensure that they are all in the same units.
At one time, the metric measure for heat was the calorie, whilst the unit for work was the joule. Today we measure it all in joules.
J is simply the conversion constant and is equal to 4.18 joules per calorie.