To put it simply, ISO is a setting which will brighten or darken your photos. When you increase your ISO number, you are making your photo brighter. Therefore ISO is a great tool to take photos in a darker atmosphere and be more flexible with your other settings (aperture and shutter speed). To clarify: the ISO is not part of the exposure.
Nevertheless, giving your ISO a boost has consequences. When you take a photo with a high ISO value your picture will contain a lot of grain, also known as noice. When you have a little bit of noise in your picture, this might emphasise the atmosphere which you are trying to capture. But too much noise might make your picture unusable. For this reason, you should only raise your ISO when you are unable to brighten your picture with aperture or shutter speed. You are in the danger zone when you reach an ISO value of 800 or higher.
Every camera has a different range of ISO values. When you are trying to take a picture outside on a sunny day an ISO value between 100 and 400 should be enough. On a cloudy day, when there is less light available an ISO value of 600 or higher might be needed. The standerd value is 100.