Nikon D780 DSLR Camera (Body Only) - Manual (Page 138)

92 Camera Controls
D High ISO Sensitivities
The higher the ISO sensitivity, the less light needed to make an exposure,
allowing pictures to be taken when lighting is poor and helping prevent
blur when the subject is in motion. Note, however, that the higher the
sensitivity, the more likely the image is to be affected by “noise” in the
form of randomly-spaced bright pixels, fog, or lines.