ISO is a unit that determines how sensitive to light the camera sensor is. The higher the number - the more camera will amplify the signal that makes the image bright. The smaller the value - the better the image quality will be obtained, but if there is not enough light in the scene that you are capturing, the result will be a too dark or even a black picture.
Balancing the shutter speed and ISO is the key to good aerial imagery. If the shutter speed is too low you will get motion blur, if the sensitivity is too high you will get low quality images cause of noise.
This is not usually a problem in well-lit situations such as a stockpile during the day, but if you feel that there is not a lot of light always remember to try to make sure that the balance between ISO and shutter speed is there without any motion blur or focusing issues.
As the cameras are getting better and better, motion blur issue is unaffected, but high ISO images are getting better because of advancing sensor technology. Raw images are getting easier to brighten up in post-production as a fix as well. As we cannot fix motion blur of an image, the priority should always be to avoid that, at the cost of using high ISO or getting darker images.