StellarEnthalpy
The enthalpy change in a chemical reaction is how much heat energy is gained or released by the system. If the enthalpy is positive, then the reaction gains energy, or is endothermic. If the enthalpy is negative, then the reaction releases energy, or is exothermic. Note that for AP Chemistry, all reactions will be conducted at a constant pressure, so the change in enthalpy is equal to the change in energy of the system.
Since conservation of energy must hold in all cases, the change in energy of the system is always the opposite of the change in energy of the surroundings. For example, an endothermic reaction will take in the same amount of energy that the surroundings will lose, and vice versa for an exothermic reaction.