To understan the effect of ISO settings (Usually this used to be fixed for a Film in analog camera). Check the following 2 photographs taken at the same time from the same Digital Camera. One with the ISO set to 100 and next is with ISO set to 400.
The above photo is with ISO 100 setting.
The above photo is with ISO 400 setting.
The camera was on the auto mode while taking this photograph. The difference is apperant. While in the night you would like to use the ISO 400 setting so that the details appear brighter.