ISO makes up one-third of the photography triangle is ISO. ISO stands for International Standards Organisation which measures the sensitivity of the image sensor.
The almost electrical interference of the pixels on the sensor is what creates noise. Noise is a random textured, almost grainlike pattern that interferes with the image quality. The bigger the number, the more likely you are to see this noise.
Low or High?
The lower the number, the less sensitive your camera is to light and the finer the grain. In bright light, a low ISO number (about 100 or 200) can be used to get a clean image at most apertures and shutter speeds.
However, in dim light, you may need to increase the ISO number to capture the image you are after (as it will allow changes in the aperture and shutter speed). However, the higher the ISO, the more likely that the image will appear more grainy.
Importance of ISO
How distracting the grain is, depends on your taste. In urban photography, the gritty quality of noise can add another dimension or element to the image. Its all about finding the balance between the components of the photography triangle to get the image you want.
By upping your ISO, you can capture faster shutter speeds and narrower apertures. Try to keep it as low as possible depending on the quality of light that you are dealing with
Be prepared that in low light situations where you would need to increase the ISO that there is going to be a risk of noise or grain.
The rules of photography are open to interpretation. Don’t be afraid to experiment to find your style.