There is lots written on ISO and it’s something that a lot of nature photographers struggle with due to their use of zoom lenses that limit the light received by the camera. ISO simply increases the sensitivity of the sensor to light, so on a dull day you can use ISO to make the image that the camera produces, brighter, bringing out detail that otherwise would not be visible. It’s all linked to getting the exposure right which is a fundamental task in all photography.
Because zoom lenses used in wildlife photography do not allow a lot of light to get through you will often need to dial up the ISO to get your exposure correct. ISO basically magnifies the electronic signal within the camera, the downside of this is that it creates ‘noise’ small imperfections like dots in the image, this seriously degrades the quality of the image, so the photographer has to balance exposure with image degradation due to noise.
Full frame cameras create less noise so you can have it set higher, crop sensors are not so good. Most wildlife photographers work on the rule of not going above 800 with ISO and have the setting on auto so it automatically adjusts. Image processing software like Adobe Lightroom is getting better at removing noise during processing but it tends to make the image look soft if to much processing is applied.