It is the difference between the temperatures of the hot and cold streams. For example, if you heat a cold fluid from 80°C up to 100°C using a hot fluid at 105°C, the approach temperature of the heat exchanger is 105-100 = 5°C. The lower approach, the higher the heating area.
In the same way as the reactor effluent temperature in the hydrotreating unit is 400 °C and the feed temperature at the heater inlet (after passing from combined feed exchangers) is 340 °C, then HAT (Hot Approach Temperature) is 60 °C.