The enthalpy of solution refers to the temperature changes in a solution when a substance dissolves in the solvent. There can be positive or negative reactions to the temperature in a solution.
A solution is a mixture of two materials called the solvent and solute. The solute is the substance that dissolves into the solvent.
Dissolving a solute into a solvent causes exothermic or endothermic reactions. Endothermic reactions create a positive change in the heat of a solution due to the energy released into the surroundings and the energy needed to break apart the solute. Exothermic reactions cause a negative change in heat by raising the temperature of a solution and ultimately expelling the heat into the surroundings.