Heat absorbed or released by a chemical reaction or phase change is passed to the liquid (usually water) in the calorimeter. A calorimeter is designed to minimize any loss of heat to the environment (well-insulated). The heat transferred to the water either increases (exothermic) or decreases (endothermic) the temperature of the water. Water makes a good fluid for this measurement since it has a high specific heat. The specific heat of water is well-documented. For this example, we'll use the standard value of 4.186 J/g°C. This means that it requires 4.186 J of energy (or 1 calorie) to heat a gram of water by one degree. We know the grams of water in the calorimeter and can measure the initial and final temperatures. With those numbers, one can calculate the number of Joules of heat energy involved in the CaCl2 reaction/dissolution (the reaction of the CaCl2 is not described, but it makes no difference).
If the calorimeter contained 100g of water, and the temperature went from 50oC to 25oC, the amount of energy that was involved is calculated:
Energy = (4.186J/g°C)*(200g)*(50oC - 25oC) = 20930J, or 20.93kJ
I don't know if the experiment with CaCl2 was endothermic or exothermic, but I used two temperatures that reflect an endothermic reaction. The temperature change is (Tf - Ti). Since this is a positive number, it means that the reaction absorbed energy from the water. A negative number would reflect a reaction that releases energy (the energy leaves "the reaction system."
