The heat given out in hydrogenation to cyclohexane is 120 kJ/mol for cyclohexene, 232 kJ/mol for…

Step 1. Benzene. Expected for three separate double bonds: $3 \times 120 = 360\ \text{kJ/mol}$. Measured: $208\ \text{kJ/mol}$. Stabilisation: $360 - 208 = 152\ \text{kJ/mol}$.

Step 2. The diene. Expected for two separate double bonds: $2 \times 120 = 240\ \text{kJ/mol}$. Measured: $232\ \text{kJ/mol}$. Stabilisation: $240 - 232 = 8\ \text{kJ/mol}$.

Step 3. Compare. $152$ is about nineteen times as large as $8$. Two conjugated double bonds gain a little. A ring of six pi electrons all sharing one cloud gains far more. That is the special stability of an aromatic ring.

Source. The heats of hydrogenation are standard values from reference tables, for example Clayden, Greeves and Warren, Organic Chemistry.