Back when photos were taken on film it was rated by ISO. For ex: 100iso would be for daytime and 500+ would be more suited to night or indoors. We still use the term ISO to measure digital sensors sensitivity to light but now we can change it in camera without having to change the film.
The consequence of increasing the sensitivity to light is the ability to take photos in darker situations at the cost of digital noise. Which presents itself as small colored dots.