This used to be the sensitivity of the film that you put in your camera and the same measurement scale is still used today.

With film, ISO 400 would have been used for overcast conditions & ISO 800 would for sports.

The trade off with a higher sensitivity setting is increased grain within your photograph. ​This was true for film and is still true for digital cameras, with improvements being made year upon year.

​​New top end DSLRs can now take very low noise images up to very high ISO ranges that go into the tens of thousands.

​As a general rule, the ISO is best kept as low as possible, although this is often adjusted to allow the photographer to obtain their desired shutter speed and aperture for artistic reasons.

​This needs to be within the constraints of the shutter speed required, amount of available light and depth of field required.