All chemical and physical changes are accompanied by transfer of heat energy. Exothermic reactions evolve heat; endothermic reactions absorb heat.

Dissolving ionic substances in water requires breaking ionic bonds and forming some sort of bonding with water molecules. These two processes each involve energy. If the lattice energy of the compound (the ionic bonds) is greater than the the hydration energy (water-ion bonding), then the dissolution will be endothermic because more energy must be absorbed to break the ionic bonds than is released by the water-ion bonding.
In SI units, heat is measured in joules (J). The heat required to raise one gram of a substance by 1 °C is called its specific heat and is measured in J/g·°C.

You'll be measuring the enthalpy of solution, ΔHs, the change in heat energy when the compound is dissolved at constant pressure. A positive enthalpy implies an endothermic reaction, the heat energy of the materials has increased due to heat absorbed from the environment.