Heat rejection is the excess heat from a cooling system which is removed by the condenser/cooling tower. Heat rejection is the total amount of heat energy which is transferred from the cool side to the warm side, plus the work carried out by the compressor.
A cooling system transfers an amount of energy from the cool side to the warm side, along with the power that is fed to the compressor in order to transfer the energy.
Most of the added energy is transferred to the coolant via the work carried out by the compressor. Thus, the amount of energy to be removed from the warm side of the cooling system is the sum of the transferred energy and the added energy. This total amount of heat energy is called heat rejection and is removed by the condenser/cooling tower.