The system is at temperature $ T_2$ at state $ a$ . It is brought in contact with a heat reservoir, which is just a liquid or solid mass of large enough extent such that its temperature does not change appreciably when some amount of heat is transferred to the system. In other words, the heat reservoir is a constant temperature source (or receiver) of heat. The system then undergoes an isothermal expansion from $ a$ to $ b$ , with heat absorbed $ Q_2$ .
At state $ b$ , the system is thermally insulated (removed from contact with the heat reservoir) and then let expand to $ c$ . During this expansion the temperature decreases to $ T_1$ . The heat exchanged during this part of the cycle, $ Q_{bc}=0.$ )