A thermal imaging device enables measuring the temperature of the target without physical contact. Thermal imaging devices can detect the smallest temperature difference of lower than 50 mK (miliKelvin) or 0.05 °C. This is called temperature sensitivity.
An easier explanation would be if there are two objects with slightly different temperature (the first object has a temperature of 20°C and the second object has 20.01°C) a thermal imaging device will detect this difference, what will result on screen in different colors.
Typical temperature sensitivity of a thermal imaging device is lower than 50 mK or 0.05 °C. The lower the value of the thermal sensitivity, the better the device. A lower value means higher sensitivity. The result is a better contrast.