NETD is Noise Equivalent Temperature Difference: basically this is noise in the images you are looking at.
Simply put this is how well (at a defined set of parameters) the thermal sensor can discern between 2 temperatures.
It is measured in Millikelvin (mK), typically from 30mK to 50mK. The smaller the figure the better the sensor can discern small temperature differences, a lower figure will generally result in a better image, with better contrast.
PARD TA Thermals are 30mk which is considered the new standard.
Excellent NETD is a fundamental that must be present. One other big item to watch out for is the F number of the Lens, often cheaper and smaller lenses are used such as F1.2, this means there will be a loss of 20% so that 35mK sensor is really 42mK.
The end result will also be affected by the quality of the lens’ F Number, the programming of the Core and environmental conditions.
Below is an image showing a lower NETD on the left, vs the one to the right – you can see far more definition in the image on the left.